All the post-TOS shows expanded and deepened main cast, leading to episodes being able to be centered on various characters. TOS had a smaller and more focused cast. It was a rarity to even have a Spock or McCoy centered episode, and I can’t think of an episode where at least one of the main trio wasn’t the most prominent character.
I think it’s just a difference in writing styles of the different times between the 1960s and the 1980s. Even look at something like STIV: The Voyage Home. Same characters as TOS, but written in the 1980s and a lot more comfortable giving various non-trio characters their own individual adventures.
Another important factor is TNG’s reliance (sometimes over-reliance) on A/B plots. The B plots were often an outlet for the ensemble characters to come out and play.
I think you can even argue that in each case there are two main characters plus a third wheel (Riker/McCoy).
But McCoy never faded from view, whereas Riker almost became a background character during the second half of TNG. They should have written Riker out of the series after Best of Both Worlds; after the character stuck around, the writers seemed unable to figure out what to do with him.
I think they did a better job than TOS did writing stories for the other crew members, but the last few TNG movies are basically the Picard and Data show
Yeah, and the problem with the Picard and Data show is that those characters don’t really have a relationship, at least not an emotional connection like Kirk/Spock/McCoy. They basically just had a professional relationship, which was fine for the series where there’s a problem to be solved every episode, and it’s not necessary to have fleshed-out character arcs. But a movie narrative needs to make the audience care about the main characters and their interactions with each other, all within a very short period of time. Picard/Data simply could not provide that emotional core.
