The X-Men are one of Marvel's longest running and most popular teams. The group has appeared consistently throughout the years, starring in multiple titles and even spawning several spin-off groups. The team itself has also been through periods of great expansion, taking in members from mutant communities all across the globe.
The point is, the X-Men and their associated teams have had a lot of members. Now, not everyone can be a Storm or a Nightcrawler or a whatever that guy with the claws is called and, inevitably, some of these new faces fall into obscurity.
Certainly, when they're first introduced, a big deal will be made of these characters. But they'll soon get pushed aside for more familiar faces. If they're lucky, some of these characters might fall into the hands of a writer who likes them and spend a brief period back in the limelight, but then they'll just get shoved right back into the background.
That doesn't mean they aren't great characters though. Indeed, often some of the X-Men's most interesting heroes are the ones who don't get used that often. Other times though, they're characters who probably deserve to hang out with the guy who can control maggots.