I don’t know that it’s any one thing, or that you couldn’t get another solo X-character book to stick. But virtually all of the key X-Men were created in the context of X-MEN–they were created as team characters, and their home title is the team X-MEN book. That’s markedly different from, say, Captain America, who for all that he’s regularly in AVENGERS has his home book as CAPTAIN AMERICA. This lends a feeling to the solo mutant titles that they don’t really count as much–that if anything truly important is going to happen to that character, it’s going to take place in X-MEN rather than their solo book. Wolverine suffers from this as well, but he’s popular enough all around that he’s been able to consistently overcome it.