I don’t really have a solid answer, and have only recently gone down the TrekLit rabbit hole, but I’d venture a couple of guesses:

1. Many of them utilize existing characters, so that might be problematic given the age of the cast and/or their availability.
2. I vaguely recall that existing characters for different series have to pay royalties to the writer who invented the character. I think this was the case for Nick Locarno and Tom Paris
3. Probably some other IP legalese mucking things up as it is wont to do

When I posted yesterday looking to get some book suggestions, many of them involved existing characters. I think the “The Fall” series is all original characters, so it would probably be a good candidate. There’s probably others, but I’m not familiar enough yet to name any more.

The actor age / availability problem is probably solvable if the series was animated, though.