While I’m typing specifically about The Good Wife this seems to happen every year. Six weeks into the new season and they’re already re-running episode #2 in its regular time slot. Wiki says it was initially approved for 13 episodes so you’d think they’d already have at least half of them in the can. What gives?
Sweeps start next week. They want to have enough new episodes for that.
Sweeps happen every year. Who are the idiots who can’t do the math at the start of the season?
They are the same people who cannot figure out beforehand that The Good Wife might be good, and successful, enough to order half a season of it. They are people upon whose judgement hundreds of millions of dollars is wagered, yet they could not figure out, in its first five minutes, that Hank would tank.
ETA: I had better examples, but they didn’t rhyme.
Am I completely fabricating experiences in my head now, or do I remember, when I was a kid (70s/80s…I’m 38), that reruns only happened with any frequency in the summer? Now it seems like the networks will toss reruns in any old time: fall, winter, spring or summer. I just don’t remember this being true before about the mid-90s.
I know summer was and probably still is considered rerun season with new episodes starting in fall, but I can’t say whether that was truth or perception.
See, that was in the back of my head when I started the thread, but you said it better. 
I went to IMDB to look up original airdates of The Best Sit-com Of All Time*. In 1975, season 4, IMDB claims that there was one episode a week from 12 September to 24 Februrary, with the exception of one skipped week at the new year.
Hm, maybe it’s just a fluke.
Dallas, season 8, beginning September 28, 1994: Skips 18 January 1985, 22 March, 5 April, 26 April. There’s no way to know, of course, if any episodes were re-run or if it was pre-empted for something else, but it still runs for three months before the first break.
Bah, I’m tired and have to go to sleep; if anyone wants to pick up where I left off feel free.
aka MAS*H
It seems to also be related to all the big sports events recently.
As costs have risen, they make fewer episodes per season, so they have to space them out more.
Actually, according to Variety, it turns out there are creative, as well as economic, reasons for fewer episodes.
Sweeps happen four times every year. The way they used to run series (twenty-some weeks of new episodes, then twenty-some weeks of reruns) you’d be okay for the November and February sweeps, but you be into the reruns by May. These days they start sprinkling in the reruns early so the new episodes will last longer into the season. The last of the new episodes run in May, with the heavily-hyped season finales at the end.
In addition to the episodes themselves getting shorter (to make more room for commercials) seasons have gotten shorter. A full season nowdays is 22 episodes, but a couple decades ago it wasn’t unusual for a sitcom to run 30+ episodes in a single season. Broadcasters realized that they could still sell ad space without the expense of producing something new every week.
I bitched about this in another thread, but I also wish they’d quit showing baseball when I’m trying to start watching shows like Fringe or Dollhouse. A couple weeks from now I’ll have found something better to do!
But which sitcoms? At random I looked up Three’s Company, Golden Girls, Sliver Spoons and Roseanne. With the exception of one season of Three’s Company that had 28 episodes, all the other seasons of all four were between 22-26 episodes.
Then I looked up some sitcoms from this decade: with the exception of the writers’ strike year, Two and a Half Men has always had 24 episodes a season; with the exceptions of shorter 1st and last seasons Everybody Loves Raymond had 23-26; According to Jim had up to 29 episodes a season…it looks like there has always been a range of numbers of sitcom episodes each season.
TV shows used to take five or six days to film an episode. Now, with larger casts, and a more filmic approach to camera angles, visual effects, and editing, it’s around ten days per episode. Therefore every so often they need to take catch-up breaks.
Think further back. Like I Love Lucy or the early seasons of Bewitched.
Eh, maybe that’s true, but I wasn’t yet a kid when I Love Lucy was on. 
Bewitched and I Love Lucy were a lot longer ago than the “couple of decades ago” you said eariler. With ILL you’re talking about the differences between TV practically at its birth and now.
Part of the change you can see with NBC’s “Endless season” concept, introduced in 2008. NBC came up with the idea that, instead of introducing all their new shows in the fall, the traditional “September to May” schedule, with repeats in the summer, they were going to introduce new shows year round. The other networks have sort of adopted the same strategy.