He’s not asking why the rules evolved the way they did, though - he’s asking why they stop when tackled. And the answer to that is, simply, “because that’s what the rules are.” Now, if he had asked “why did football and rugby evolve separate rules for tackling” then that’d be a different question with a different answer.
If he’s downed but not held then he can get back up, but he cannot free himself while he’s on the ground - so if he’s downed and held then he must either pass the ball immediately, place the ball on the ground within reach, or just plain let go of it.
Because the game is free-flowing enough without it - whereas, AIUI, with gridiron in the pre-forward-pass days you just had one basic tactic that got rinsed and repeated the entire game, and adding the forward pass revitalised the entire game.
True, I’d forgotten that although the player can break a tackle and keep going or pass the ball so play continues. Just saw some league highlights on TV and there was plenty of running and bursting through tackles and passing. Granted, these are the best plays and are the exception to the normal progression up the field which is very like gridiron.
Yes, much like gridiron will have a steady diet of shoving the ball up the fullback’s jersey and making a yard or three through sheer muscle, League often features plenty of attritional grinding through the big ball-carriers. But both have some alternatives from time to time which open the game up.
I used to watch televised League sometimes in the 1970s and on the whole it was a duller game then - but that was mostly the bread-and-butter stuff on a wet Tuesday evening in an English winter, and not conducive to very exciting play. The modern game may be more adventurous, or it may be that now I’m only ever seeing the end-of-season big matches and internationals since TV schedules are filled with other stuff. Back then it really was, on the whole, a case of grinding forward for five tackles and then booting it down the park and hoping the other side made less ground on their return trip - not unlike, I suppose, gridiron in the pre-pass days, except for the tackle count.
Of course there no reason why the US shouldn’t have its own version of football. Clearly the game works. Ireland has Gaelic football and Australia has Australian Rules Football (AFL) which is hugely popular.
What puzzles me is the stop-start action on the field. Very little actual play takes place as evidenced by a Wall Street Journal analysis. They calculated there were 11 minutes of actual play with the rest of the time being setting up the teams and running the clock down. Seems hard to believe but when I used to follow the Denver Broncos, an actual play only took seconds - then it all stopped and for quite a while sometimes.
LOL you’ve just described rugby union in the dim days of my boyhood. Black and white tv coverage (sent over on film in an aeroplane) of grim faced All Blacks at Llanelli and Twickenham. Rucking with boots. Mud and blood. They were hard men all round.
It seemed to me that league introduced a turbo to their game in the early 1990s and really moved up a gear. The game sparkled and even union devotees switched codes. Still, I went to a game a few years ago and discovered that most players just stand around while a small group in the middle do the crashing. Not much action at all and quite disappointing. Television hides this because it zooms in on the middle.
Rugby union in the meantime saw the writing on the wall, rules changed and the game became transformed. IMHO it is exciting and sufficiently complex today to be a worthy international sport.
There was a really interesting article on what rugby can teach us about game design in the Guardiana couple of days ago. The basic point is that games with no restrictions are boring and degenerate. The common ancestor of all football codes was “hugball”:
The limitations imposed by rules are what make games interesting. Take Mario, as the article suggests - if he could jump as far and high as the player liked the games would be boring. It’s the fact that he can only jump *so *high, and *so *far, that makes the game a challenge and winning an accomplishment.
American Football is a game about the precise execution of pre-planned tactics, and how well each team can select and execute its game plan in the face of the other’s. That’s what the rules are designed to achieve. You might personally not like it, just as others don’t personally like the low scoring in football (soccer). But changing the rules to make it more like rugby union when rugby union already exists seems like a fairly pointless exercise.
Yeah, but shouldn’t it be EVEN MORE free-flowing? Why are you opposed to free-flowing games? 
As I posted elsewhere, the stopping and starting is a feature, not a bug. It’s not a quirk of our version of football, it is the ESSENTIAL rule. The four downs, the line of scrimmage, forward passes. Everything is predicated on play stopping and restarting. Stop thinking of it as a weird thing that keeps happening that stops the game. It IS the game.
The separate question of whether too much time takes place between downs is valid. It could be tightened up. The WSJ study was flawed, though, timing from the snap of the ball. Plenty of purposeful action happens between the break of the huddle and the snap.
The amount of time between downs has been shortened, on average, as more and more teams (especially in college) run hurry-up offenses. Some people actually perceive this speed-up as a problem, as faster plays mean more plays, longer games, and greater chance of injury.
That depiction of gridiron / American football is, for the most part, out of date. Nearly all NFL teams pass significantly more often than they run now (and have for 20 years or more); over the past 30-40 years, numerous rules changes in the NFL have specifically been made to open up the passing game.
Granted, many NFL passes are now fairly short, high-completion-percentage plays (with those short passes serving the purpose that run plays used to), but “three yards and a cloud of dust” hasn’t been a part of NFL offenses for a long time.
In the college game, there are still a few major schools who favor traditional, run-first offenses – my alma mater, Wisconsin, is one of them – but most major colleges now run a “spread” offense, which features a lot of passing and option running by the quarterback.
Going back to the “take a knee” thing: a few years ago I was at a college game where, on the last play before the half, the quarterback first acted as though he was going to take a knee, but didn’t, and attempted to drop back to pass. The refs whistled it dead.
Afterward, I looked it up, and the ball is indeed dead upon the quarterback “simulating” going down to one knee. The rule makes sense when you think about it: a kneeling QB stops the defense. Kneeling makes defenders hit the brakes, because they don’t want a penalty, and the fake-out QB gains a couple of extra seconds with no one charging him. That shouldn’t be permitted.
I believe Mark Sanchez did that once, and got flagged for it, but I can’t find a clip on it. I guess it’s so far down on “Mark Sanchez career low-lights” that everyone has forgotten about it.
However a fake spike is a perfectly legal play, used famously by Dan Marino and more recently by Aaron Rogers. And the spike is only a legal play because there’s a rule specifically making it not intentional grounding.
Although you can’t fake it and then actually spike it. Caleb Hanie (ugh) did that once. It’s an intentional grounding penalty.
I don’t know why, a recent RWC Eng v Wales match had 20+ lineouts, 10+ scrums, 23 penalties, and 12 penalty goal attempts. Seems reasonably typical and a lot of stoppages in 80 minutes.
Thank you. I’ve seen a few televised games from time to time, and never a full one, so my opportunities for education on this are vast. 
Good point and it slows the match down substantially. In fact there have been mutterings from the Springbok and All Black coaches complaining about it.
In the southern hemisphere stoppages are shorter, referees don’t muck around. Even when there is an injured player on the field play carries on unless the injury is clearly serious.
I myself have been surprised at the World Cup stoppages and wish the games were faster but apparently its a northern hemisphere thing.
Reminds me how my father, a diehard baseball fan, would complain about stopped play in ice hockey. That every pitch was a stoppage didn’t matter; an offsides or icing whistle was too much for him.
That’s a total of 65 (assuming you counted the penalty tries separately) stoppages, plus one for half time and one at the end.
The typical NFL game features 130 plays from scrimmage - meaning 130 stoppages in play - plus a whistle at the end of each quarter and one bonus one for the two minute warning at the end of each half.
So even a huge outlier in rugby has half as many stoppages of open play as the average NFL game, which is only 60 minutes long (yet takes about three hours to play). Amazingly, very little of that time represents commercial breaks (though god knows there are about a dozen too many of those); the average high school game (48 minutes of “clock time”) takes about two and a half hours to play.
I’m a huge American football fan, but it would be so much better if they fucking got on with it.