Series television, especially when presented as a continuous narrative rather than in an episodic format, offers a unique opportunity for in-depth character development. Unlike films, which are often constrained by their shorter runtime and tend to focus on a small cast of principal characters, television series have the luxury of time. This extended format allows writers to explore their characters more fully, giving audiences the chance to see their growth and evolution over multiple episodes and even seasons.
Apart from TOS, which was largely episodic at the time (1960s) with one or two references to past episodes at most. Then came the 1979-91 movie series where there was quite a lot of character development (chiefly for the big three), with an intertwining arc/continuing epic (for the most part). TUC did manage to set up and sell in 2 hours a formidable opponent in General Chang, in a trope that isn't " revenge on Kirk, moohahaha", that also concluded the intertwining events that started in TWOK-onward. 2-4+6 are impressive. 5 doesn't quite fit without some headcanon, but 5 is more like what TOS circa 1968 would feel like on the big screen and with shinier sets. It's an interesting film, but that's too far a tangent.
But TOS's movies are the odd ones out, only because they had the established TV show characters to play with. (The TNG movies definitely didn't do the same continuing saga, which some fans were probably expecting at the time... oh, one flick has "The Dominion" mentioned, but they could have said "The Dustbunny" and it'd be just as pointless.)
TV from the 90s onward did introduce the type of character development that couldn't be done in movies*. Having 26 episodes to play with per season helps a lot, though PIC season 3 is a great example of having ten and still hitting it out of the park (IMHO).
With big movies feeling more fizzled than fantastic, it's sorta easy to see why tv shows now, despite fewer episodes, try to feel more cinematic - with HUGE budgets. I just wish they'd find hues other than teal and orange at times, but as David Gerrold once alluded to, "a good story survives anything". I think he'd said that about "Land of the Lost" (1974), another show that's impressive for (a) the time and its budget, and (b) ostensibly being a kid show and yet season 1 feeling transcendental of that. There are some TNG connections to that show as well, if you look through the end credits of various episodes...
