Star Trek has a rich literary background, with hundred of stories that could be used in some capacity in a movie or television episode. Not all of them should be, of course, but when you have something that is for example a best seller, how come no one is approaching that as a potential idea to use?
Tie-ins are rarely "best sellers" in the way that term is generally understood -- and even a best-selling book has a tiny audience compared to a moderately successful television show. The audience for Trek prose or comics tie-ins is at best 1 to 2 percent of the audience for the shows and movies. There's certainly no reason they couldn't borrow an idea from the books if they were aware of it and considered it creatively useful, but it wouldn't be done to court popularity.
I get that Star Wars didn't (and still doesn't) have the plethora of TV media to engage with, but at least those show creators know when a popular character like Thrawn or a concept like kyber crystals should be incorporated. Couldn't Trek do the same?
First off, there is no "should" here. It's not mandatory. They use ideas because they like them and find them useful, not because someone's compelling them to.
And second, yes, of course Trek could if the creators wanted to. There isn't a single blessed thing stopping them. They can do whatever they damn well please, because they're the ones making it all up. They just don't need to, because they have plenty of their own ideas. And as I think I've explained earlier in the thread, Star Wars is a franchise whose supporting merchandise (including books and comics as well as toys, games, etc.) has always been proportionally more important relative to its screen content, because up until recent years it was mostly a film franchise and thus had a limited amount of screen content that only came out intermittently. With Trek, the tie-ins have not been so prominent. It's just not reasonable to expect Trek and Wars to work exactly the same way. It's apples and oranges.