Well there's just nothing keeping them from continuing the idea. Why not keep going now?
Because people really don't care about those characters. When I pick up a TOS book, I want to read about Kirk, Spock and McCoy.
Much like I don't really care about Elfiki, Chen and Choudhury in the TNG books, I have no emotional investment in them.
I find that if you write a character well I will care about them. So I like in the DS9 relaunch I came to care about Shar, Ellias, Prynn and so many others. In the TNG relaunch I was heart broken when Choudhury died; I want to know more about all the characters.