Sulu, Uhura, Chekhov, and Scotty were named cast members it's true. But they didn't really function like the secondary characters on the later shows. First, they were absent more frequently (Scott, for example, was absent from 17 episodes). Secondly, a lot of their dialogue was solely related to their role on the ship. In most cases when they actually did have a weightier role (like going down on an away mission) a "guest star of the week" could have done it just as well. In terms of plots...I mean, Scotty was a plot device in Wolf in the Fold. And Chekhov got to have a space hippy ex girlfriend in Way to Eden. But that's it. We never got an "Uhura episode" or a "Sulu Episode."
It's not fair to bring up Yar because she left so early on. Code of Honor arguably was a Yar-focused episode though, as crappy as it was. Troi got her fair share of episodes across the entirety of TNG, even if she was horribly misused as a "bridge bunny" up until the end of the series.
Someone claimed that the writers couldn't figure out how to stretch a plot over an hour. This is false. The problem is that good plot ideas will seldom involve every single primary/secondary character on an ensemble show. What Trek should have done is just not have the actors appear if there was no story reason to do so, and even out the screen time with having an episode focusing on them more heavily later on. But for whatever reason, they felt like they couldn't do this, which is why if you watch say a DS9 Ferengi episode there will almost always be some random scene thrown in at the opening and/or closing where Kira and Sisko are talking about something only tangentially related in his office. The shows were transparently scripted in order to showcase the actors, rather than the stories.
Edit: Note that Discovery does this too. Consider all of the weird roles Tilly was thrust into over the course of the series - from becoming the "spore expert" who healed Stamets to Captain Killy. To a significant degree, this seems to be make-work for the character so as to keep her relevant once the show drifted away from its initial "lower decks" conceit.