Schedules and budgets.

Berman-era Star Trek was written as "we start the season here, we end the season there." Some actors had prior engagements and were unavailable for all the episodes; some actors had conditions in their contract that said "at least one episode in this season is gonna be about me." The main throughline had certain set-pieces built in; this was where most of the money for the season was gonna go. So you take all that, you put it in a Gannt chart and you recognize that you've got seventeen of the twenty-six episodes you're contractually bound to make, and you've got four million dollars left over (TNG was infamous when it came out for being the first syndicated series that cost more than a million dollars an episode). So now you've got nine episodes and half your average budget and Patrick Stewart is playing Scrooge on TNT and Gates McFadden is outtie because she's sick of Maurice Hurley rubbing his dick on her in the dressing room which means when you go to the pile of spec scripts you have from writers on and off the show, you can cross out everything with Picard or Crusher. And what you're left with might be great, might be terrible, but can certainly be executed in a way that doesn't fuck up your through line.

Not all shows are done like this. I got to talk to David Kring right as Heroes Season 1 wrapped. I asked him how much they knew about the end of the series when they started, because S1 is fuckin' intricate. He blew me away - they knew nothing. They fully intended to kill the cheerleader three episodes in. HRG was a bit-player. They thought it was going to be a show about the Patrelli brothers. But as they wrote, they came up with this thing and they rolled with it. But they ran out of money - in the finale, Patrelli was supposed to throw a bus at Sylar but they didn't have any money for that level of SFX so it ended up being a bus stop sign.