A lot of sci-fi (at least where TV/Films are concerned) keeps getting too bogged down in what it thinks that it should be, and doesn't actually try to explore new possibilities or expand much, which generally means that the quality of sequels progressively gets worse, and the show ends up being a sort of even mush vaguely resembling the original.
The main example I could think of is probably Star Trek. It's too fixated on everything as it is, so even things that are supposed to be radical changes just re-establish the status quo with a new coat of paint. A radical show with radical viewpoints would never take off, as newer iterations would try to emulate the success of the show, and keep to the old.
It's part of why later Star Trek shows seem to be a bit more conservative, by comparison. Sure, values have changed since the original show, but the level of radical progressiveness has also gradually wound down too. Compared to the original show, which tried to push things from all angles, something like Star Trek: Discovery would seem almost conservative. Most of its more progressive elements are fairly standard for the time period it is set in, rather than pushing the envelope like the original did.
Similarly, all the shows end up trying to emulate the same formula, and even the same rough starship design. The Enterprise was originally specially designed and built to seem future-y, but many other of their starships since them seem to just be iterative designs on the original. Even one of them set 900 in the years in the future seems to have almost identical technologies, polities, and culture as one set in the 24th century. The visuals are different, but everything seems to be effectively the same under the coat of paint.
Not having that baggage is probably why up-and-coming shows, like The Orville, tend to be able to get away with more, since there isn't a previous Orville that it keeps trying to recapture, just yet, which should mean that it gets more leeway.
From a non Star Trek standpoint, it's also rather happened to Star Wars. The newer films are just trying to recapture the older films, rather than expand into their own thing, to the detriment of the films as a whole. The latest trilogy seems like a rehash of the old ones, down to having what is basically another death star, Rebellions, Vader-ish Masked Sith Lord, and Friendpatines.
I don't really have much of a solution, besides wanting the shows to just branch out more. I think Star Trek in the 32nd century should have gone with a brand new slate, where everything was different (from both an ideological, political, and technological standpoint), and the 23rd century ship that ended up there would be woefully outdated, not just on paper, but with the technology it was fitted with.
Star Wars has a bunch of interesting things that it could run with, such as the aftermath of the major wars, where the Rebellion is now having to deal with multiple smaller wars from various factions under the splintering empire, or have to secure its place in the resulting power vacuum.
One show that hasn't succumbed to this as much is Doctor Who, but that had a major revamp in its 2005 revival which drastically changed the nature of the show itself. Still, it doesn't seem to be particularly immune to it either. Behind-the-scenes, they're suddenly going back to the old composer and old showrunners, and the main character doesn't seem to evolve too much beyond "conflicted, but brilliant and eccentric hero". It also seems to be slowly settling into its own ruts, as well, with the most recent run rather resetting a redeemed villain's character development suddenly.
As a slight tangent, I also feel like that considering the messaging of the show itself, there could be quite a bit of interesting mileage that could be achieved by having a companion who is a species that is normally an enemy. Maybe something like a Dalek.
