It was the fact that he stole Hick's act (word for word in some cases) and stage persona, giving rise to the joke:
"Why is Denis Leary a star while Bill Hicks is unknown? Because there's no cure for cancer"
( ,
Thu 7 Jan 2010, 13:35,
archived)
"Why is Denis Leary a star while Bill Hicks is unknown? Because there's no cure for cancer"