That just boils down to a lack of imagination imo and falling back on things that once worked rather than doing something new.
Star Trek is the perfect story engine. It can be anything and everything. It's not limited in what it can or cannot be. It can be speculative fiction, social commentary, action-adventure, space western, time travel, and even Hard SF. But for some reason, what Trek can be becomes narrower and narrower, both by those making it and us fans.
Hell,
@Mark 2000 has proven you can go to new and more interesting places in the Trekverse with his fan webcomic:
http://trekcomic.com/
So I don't buy it can't be done. It just no one has really wanted to for whatever reason.