Changes in Showrunners. Sometimes it's a turn for the better, sometimes it's a turn for the worse. It depends.
TNG, I really hate to say this but: the less Gene Roddenberry was involved, the better the show was. Maurice Hurley being the Showrunner was a step up, but Michael Piller was an even further step up.
Ironically, I think VOY got better once Michael Piller stepped down. I think it got worse when Jeri Taylor stepped down. Then I thought it improved again in Season 7 with Ken Biller as Showrunner. So, I preferred VOY seasons under Jeri Taylor or Ken Biller over VOY seasons under Michael Piller or Brannon Braga.
If Manny Coto had been the Showrunner for ENT from the beginning, I'd consider myself a fan of the show.
We never got to see DSC under Bryan Fuller, so we'll never truly know what it would've been like. We can only speculate. DSC under Gretchen Berg & Araon Harberts versus DSC under Michelle Paradise could be debated forever. I have no stance at the present moment. Ask me again in a year, when I've had some distance from the series as a whole.
Having read Patrick Stewart's autobiography, Making It So, I know that Paramount mandated that PIC Season 3 be a TNG Reunion. Patrick Stewart said it was something he didn't want but softened on that position by the time the third season rolled around. So, my opinion was the same as his and it changed along the same line as his at around the same point ("Nepenthe"). I know some people like to use Terry Matalas as a scapegoat, but PIC Season 3 would've been a TNG Reunion whether it was Michael Chabon as the Showrunner or Terry Matalas. It's what the studio wanted and it's what a lot of people wanted. But I do think a Michael Chabon PIC Season 3 would've been different from the actual PIC Season 3. So, once again, the difference in Showrunners matters.
Back in the '90s, I told someone who watched DS9 and VOY but didn't know all the details like I did (and do) that the two shows had different writing staffs and he said, "That's very obvious." You didn't even need to be familiar with all the names in the credits to be able to tell.