you see, hollywood is an industry.
so, that's why they do these things. free tv is run the way that free print publications are run--advertisers pay for space. well, in tv-land, they pay for time.
so, lost is a hit, the time for commercials is limited, the time costs more to the advertisers. loosing a hit show means loosing money--what if it's replacement isn't a hit? and so on. . .
so that's why.
and they're also trying to revamp the whole 'seasons' thing into some year-round schedule, but also trying to make it fit with the old, secure contract schedules that writers, actors, directors and producers are used to. they're used to getting paid for 18 or 22 episodes to be shown from sept through may (with a break around christmas and a break from march to july--thohugh some producers working on pilots have to work super hard march through may). with the new system that they're trying to create, you might need somewhere around 30 or more episodes a year, running on an all-year, staggered calendar (with 'fall' and 'spring' start ups) for both production and showing.
this also messes up the game for advertisers, in that what makes a hit means different things at that point--how many episodes, how many people are watching--and tivo has really made a dent into this. so, how much should that time/space cost? if the show's a hit--how do they know? which numbers are coming from where? the fear is that the staggered time slots will take away from fall 'hits' thus deflating the advertisers investment mid-year.
and then there's those prod-crews to think of--you might need more than one or two, depending upon the unions, how many hours people can work, and how well the different crews may or may not work together--and then the filming schedule and how it affects the actors--always a big question as stars keep the show a hit, and the hit factor keeps the advertisers happy.
so that's the reason.