Easy answer why: it's not profitable enough for them. RPG settings based on historical cultures are BY FAR my favourite thing in all of RPGdom. But I'm aware my tastes are relatively uncommon. Which means such things don't sell terribly well, which in turn means while it can be profitable for a small publisher to put out such works, it's simply does not generate sufficient sales for it to be worth D&D's time and effort, when other products they can invest in would sell better.
Such works were ALWAYS in by far the minority in the world of TTRPGs, even when TSR was doing it. Even for D&D alone, there was one book based on Vikings and bloody hundreds of books for Forgotten Realms, for example. So that hasn't changed either.
As for all of fiction, you're again demonstrably wrong about that. Here's an article discussing some recent novels based in historical times, some of which are VERY recent, and the article itself is from 2020, which demonstrates that these works are socially acceptable. I picked this one simply because it was the first google result. Even a cursory search will provide mountains of evidence that your claim is false.
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So no, there is no oppression by "forward-minded types" making it impermissible to talk about the past. Every claim you've made about that is demonstrably false, as demonstrated above.