Deimos Anomaly wrote:
(According to Daniels they eventually joined the UFP.)
Obviously the real life reason is that Enterprise was made after those other series.
Other than that though I wondered if anything had ever been put in canon (or even quasi-canon, like a producer or writer or one of the ST bigwigs making a statement in an interview or what have you) by way of explaining why they are never seen "again".
The Xindi get occasional mention in the 24th century post-series novels. There was a Suliban member of the DTI in Watching the Clock
As for why they weren't ever seen on-screen, it's a BIG galaxy. We never saw the Remans until Nemesis, although they were supposedly part of the Dominion War. We never see many of the cool background aliens from The Motion Picture, The Voyage Home or JJ's Star Trek in any of the chronologically later series' or movies either.
Back to the novels for a second, Spock's World
mentions that the Federation has had contact with 800+ alien races. They can't all be on screen all the time!