Because Generations is. like all TNG movies, a rather stupid movie.
The Nexus is just a badly done plot device with more holes and most of us would rather NOT remember that stupid movie, besides time travel? We already had a featured episode with that, not to say the Klingon War have even more time travel, its over and done.
I personally think it might be a cool little "1 week event" one of these days as the Nexus passes through Fed space again, that's about it. The Nexus.. I personally don't see much story potential in there.
The Nexus is not quite time travel.. more time a time trap, but still I'd think it would only be worth seeing it move around sector space and maybe have a unique 1 week only special mission relating to it.. that's about it though.
"We are the Borg. We will add your biological and technological distinctiveness to our own. Resistance is futile."
The Cube Assimilating Your Ship Right Now
Bill Shatner VOICE OVER....update on Captain Kirk...lots of potential there, even if only for a limited time. It's not like Shat would turn it down. He would probably get a kick out of it, and probably would attract more players to the game.