Well I think it's all about the circumstances of the titles they're in. And none of them have solo titles because Marvel doesn't feel that they have the fanbases to support their own ongoing books.

The Young Avengers don't have an ongoing book but they do constantly get miniserieses (I think the next one was recently announced. Avengers: Children's Crusade with Heinberg and Cheung).

Runaways was put on hiatus because of low sales.

Northstar is a part of this "revolving cast" in Uncanny. But he is very much the definition of window dressing, but that's par for the course with Matt Fraction on Uncanny, Northstar gets about as much focus as any character who isn't Cyclops, Beast, Emma, Pixie or the X-Club. But at least Karma is on New Mutants, and Shatterstar and Rictor are in X-Factor.