I'll never get why US television schedules are so needlessly complex and drawn out.
Probably inertia, to a large extent. In the '50s, a TV season was typically well over 30 episodes, running pretty much continuously for nearly 3/4 of the year from fall to late spring, with weeks off only for production delays or special events. Over the years, as it was discovered that audiences were willing to watch reruns, the number of new episodes per season diminished -- typically 26 episodes by the '60s and '70s, down to 22 for most shows by the '90s, now sometimes even less. But the 9-month seasons continued.
Then there were the "sweeps" conducted by ratings services several times a year, typically November, February, and May (and probably August too, but that was during the break). Because those were the times when the ratings services assessed show performance in order to provide that information to advertisers, the networks concentrated their new episodes around those sweeps periods, so the seasons had to stay spread out over the year even as the number of episodes diminished, and thus seasons got broken up into three or four segments with gaps between them. Although cable shows, which tended to have shorter seasons, evolved a pattern of breaking them up into two half-seasons instead. These days, we're in transition as the old models break down and networks experiment with new patterns and year-round scheduling, so things are a little unstable.
(Really, the whole idea of sweeps seems like a bad idea to me. The goal is to get statistical information on show performance, but since the networks know when the sweeps are coming, they concentrate new episodes around sweeps months and employ ratings-grabbing tactics like airing specials and having series arcs build up to big, important climaxes, and thus the ratings figures that result are artificially skewed and not statistically representative of the shows' overall performance. It seems it would've been more scientific to perform sweeps on a secret schedule so that the observation wouldn't bias the result.)
But sports seem to have a lot to do with it too. There are some unfortunate timing alignments between TV seasons and sports seasons, and that can interrupt shows' schedules to their detriment. I remember a number of instances where a show would premiere, run for about three weeks, then get pre-empted by baseball for four or five weeks, and have trouble finding an audience when it came back. I think that smothered a number of promising shows in the crib. They should've just rearranged the broadcast schedules, but I guess they felt pressure to premiere their shows at the same time as the other networks that didn't carry baseball. (That's another thing that's changed -- the pattern of a uniform season premiere date for all networks doesn't really apply anymore, now that we tend to have more year-round schedules.)
