Nothing really, he was an action-movie star in the 80's. A comedian (can't remember who) made some jokes about being tough like Chuck Norris and it stuck, kinda like that comedian that does the "You might be a red-neck" thing, or Johnny Carson and his "How hot is it" jokes.
I'm telling you folks of the three "Chucks" only Manson wouldn't last. If he were out living at Spahn Ranch then the two other Chucks, Liddell and Norris, might be in deep doo doo because then all Manson would do is get one of his acolytes to kill for him. But Liddell could easily make Norris "upchuck" in the Octagon. Even though Liddell is retired now isn't he?