I think CLAMP had a falling out with Asuka. At the same time X "went on hiatus" CLAMP's other series running in Asuka, Gohou Drug, also ceased publication. (I was subscribed to Asuka at the time and greatly miffed by both of them disappearing).

Nekoi still loves GD though, she often laments on her personal board about not being able to draw the manga, and draws the characters often on CLAMP's oekaki (http://kasumitodo.altervista.org/downloadarea_site/mangaka_clamp_album.htm).