I think the non-political link was the fashionable and narrow minded desire for subversion by the writers and creators, probably because of things like Game of Thrones and other edgy kinds of shows that became popular last decade. The problem is, these three properties are very traditional, largely upliftings male dominated stories. For all of their complexities, The Doctor is a hero, the Star Trek crew are heros, the Rebels are heros - and they have logical, well known tropes. These universes don't really work on that deconstruction unless you're willing to make characters complicated. As people upthread have said, these politcs would be much more tolerable if they were better written - but the politics of the new generation prohibits classic tropes and complexity as oppression. Women can't be weak, villains can only be misunderstood or paranaturally evil, race and history is just a colour, and all of this must be presented as subversive because it's cool. In short, the people running these very straightforward franchises cannot understand them, cannot write stories or real characters, and believe that subversiveness will make them fresh and exciting. All three of these franchises still believe that strong women are this new and exciting thing that nobody's ever seen before, because that's their least offensive innovation.
I don't think there was a conscious agenda to ruin or neglect these franchises, but since so many of these people were only interested in revisionist politics, a white and black worldview and though subversion came across as hostile, which then pushed the creators of these shows into a reactionary activism because they didn't understand that they were incapable of disguising their politics. The urge to subvert meant everything got tainted - all three of these properties new entries completely wreck, or invalidate the older stuff because the desire to subvert was that shallow. Even Pablo Haldgao's official Disney canon and the Discovery timelines contradict themselves, so why would anyone bother doing a comic or book for a universe that would make it obsolete?
This isn't neglect, or even really spite, this is mass catastrophic failure of imagination from extremely narrow minded and incurious people who don't understand what's going wrong. When they do try and course correct and can't be subversive, the only thing they can do is nostalgia baiting, because they do at least understand that the old things are popular. The canon and history is 'problematic' or whatever, so it's essentially pick and choose, which is why so much of it seems like rushed fanfiction. It's probably the only reason The Rani and Omega were brought back, because given how long they've been away, they didn't do much.
Doctor Who took longer because it's modern audience was much more diverse and progressive and it played with time and rules constantly, which sort of illustrates how incompetent you have to be if it's still a breakneck, confusing, contrived story. I never had a problem with the diversity in Doctor Who, even during the Jodie Whittaker run: it was the completely current thing political leanings in their stories was impossible not to notice. A time travelling white supremacist. The Indian Partition. AI workers replacing people, because nobody had thought of robots in science fiction before. They weren't doing subversive stories, the subversion became the story. The characters were tokenised and made passengers in a political lecture. That last moment proved that Gatwa and Whittaker could have done wonders if they were allowed to be characters.
But of course that's not subversive and current enough, so Russell comes in and makes Davros an old guy, Issac Newton black, puts some trans people in, and brings the most popular Doctor back and reduces him to a stereotype of a black metrosexual, because as he's admitted, that's literally all he can write about. Not to make his politics appealing, or even make a point, it just becomes subversion for it's own sake. Because his politics demand that's all he can do.
