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.

I was talking to my cyberpunk loving, DS9 suggesting friend again last night and he told me Voyager can be a hard watch if you care about consistent characters. He said the Captain seems to have a new personality every week. Sometimes she's all about the rules, sometimes she says damn the rules. Sometimes she's very encouraging, sometimes she's cold and harsh. From what he described, and from what you're describing, it sounds like either writers are being rotated in and out and don't understand the characters or the producers solicit scripts and then try to mold them to fit the show.

I hate to compare and contrast westerns again (but hey, I'll stick with what I know I guess), I have a theory that the people in charge of the shows would often just buy scripts and then re-write them a bit to fit the characters and scenery of a show. A lot of the times plots would be so simplistic and stereotypical that anyone could easily take place in The Rifleman or Gunsmoke or Laramie without much adjusting.