In 1996, Marvel had some of its titles participate in "Heroes Reborn", a crossover that allowed Marvel to relaunch some of its flagship characters such as the Avengers and the Fantastic Four, and outsource them to the studios of two of the former Marvel artists turned Image Comics founders, Jim Lee and Rob Liefeld. The relaunched titles, which saw the characters transported to a parallel universe with a history distinct from the mainstream Marvel Universe, were a solid success amidst a generally struggling industry, but Marvel discontinued the experiment after a one-year run and returned the characters to the Marvel Universe proper. In 1998, the company launched the imprint Marvel Knights, taking place within Marvel continuity; helmed by soon-to-become editor-in-chief Joe Quesada, it featured tough, gritty stories showcasing such characters as the Inhumans, Black Panther and Daredevil.
What caused Image Comics to discontinue the Marvel crossover?