Just look at the immense popularity of the game ! Is it that soccer is simple to play ?
Any other reasons ?
Also the fact that it is very cheap. All you need is a ball which can be made from rags.
It might be cheap to play in its most basic form, but it’s not cheap for the millions who spend a significant portion of their earnings just to watch people playing the game.
There are two factors.
One is the ease of play thing.
The other is that, for the most part, the basic rules of the game are simple and easy to understand (even now).
It meant that players could pick up the rules quickly and then get on with the game without worrying about remembering whether what they were doing was illegal according to some arcane rule somewhere, and spectators could watch and understand. Additionally, because the game requires so little specialised equipment most of the spectators had probably played it.
Until relatively recently there was only one law with any judgement complexity on the books, and that was the offside law. Then they started introducing rules that put some arbitrary limits on the game - but in most cases kids can just play a kickaround game without worrying about those.
It’s aesthetically pleasing. The “Beautiful Game”. Subjective, yes, and I know some people would disagree with that, but it’s a big part of why I like the game.
There is almost always something happening versus American sports like football and baseball, there there are long periods of inactivity broken up by short bursts of action.
All players are involved in the game most of the time (save maybe the keeper) and any can score; it’s a bit more democratic than many American sports that way. The closest North American sport in this regard is hockey.
Minimal equipment is required. Even if you don’t have enough people to play a game, it only takes two to kick a ball back and forth and even with just one and a wall you can spend many hours. Or just juggling (or trying to).
We also have football chants, which enhance the spectator experience. Do you have this sort of thing in US sports? I can’t say I can recall any, except from those from the cheerleaders.
football chants…holy crap…do you have football chants!
I wish we had football chants instead of the twatwaffle cheerleaders.
Several action movies (The Crusaders, Arn: Empire at the End of the Road, etc) have recorded their action sequence roars at football stadiums in the half-time break. I think they did something of the same with LoTR, except that was a baseball match, I think?
ETA: As a VIF fan (Vålerenga, in Norway) I’ve got to admit, being on stadium and singing “Vålerenga Kjerke” is pretty much the closest thing I’ve had to a religious experience. 
I agree with the other answers: it’s cheap to play (no expensive gear required) and pretty much everyone feels like they’re contributing pretty much all of the time.
It’s also democratic in the sense that you could be 5ft nothing and the best player in the world if you have the skills and fitness.
True. You have all sorts of types playing the same sport, for instancethis guy and this guy are equally top rated.
About the simplicity - according to Nutt, a character in Terry Pratchet’s Unseen Academicals, it’s so simple that any child can play, but the strategy of playing well can take you a lifetime to learn. That’s paraphrased, I couldn’t find the quote.
I think this is true of any sport. I don’t think we need to look any further than the fact that kids play it, so they learn to like it. I played baseball as a kid, so that’s my sport of choice. If you talk with my football-fan friends, you’ll mostly find that their fathers watched it consistently or that they played it in school.
It was at a cricket match in England; significantly, it was a Test match between England and New Zealand, in which Peter Jackson would have obviously taken a nationalistic interest.
I don’t suppose most followers of cricket went to the game expecting to make Uruk-Hai chants during the lunch break.
I think there’s one other very big factor: the game is enjoyable and challenging at every skill level, from 11 year olds to the best professionals in the world to 40 year olds playing rec league (soccer actually suffers a bit a the highest levels, IMO, because it gets a little too hard to score, but it’s not a fatal flaw).
It’s not trivial or obvious that any sport would have this quality. Take for instance racketball. I played it a bit with some friends in high school. It was great fun while we were learning, but once we got basic competence, it was pretty boring with not much strategy or variation, compared to say, tennis. Conversely, something like ski-jumping or unicycle-basketball or whatever might be incredibly interesting and challenging at a high level, but it takes too much skill/practice to get to that point, so there are no casual/recreational players.
And finally (and I think least importantly) soccer also scales reasonably. It’s not quite the same, but you can play a fun game 5 on 5 with a small field.
I don’t think that’s all of it. My dad was an association football fan in the sense that if Liverpool were on TV he’d watch - but he was a huge motorsport fan.
I grew up loving motorsport, but now that I’m grown I hardly watch them (mostly because in the US everything involves going around in circles). I didn’t grow up with American football, but I love it. And I love association football.
That’s a pretty big if.
Hey guys, anybody can be the best soccer player in the world… provided you’re the most agile and dexterous person in the world.
Skill, agility, and physical fitness are every bit as much about being physically gifted as being 6’4" and benching 400 lbs.
Is this close enough 
Hail! to the victors valiant
Hail! to the conqu'ring heroes
Hail! Hail! to Michigan
The leaders and best!
Hail! to the victors valiant
Hail! to the conqu'ring heroes
Hail! Hail! to Michigan,
The champions of the West!
I concur.