Chow told the AP that he had abandoned directing the film in order to make a superhero comedy starring Jack Black. "If I direct The Green Hornet, the superhero comedy will have to be delayed for two years," Chow said. "The timing might not be right for a superhero comedy in two years. And I want to make a movie based on an original idea."
Presumably, directing Black might also interfere with his Kato-playing duties -- something Chow confirms by saying that continuing in the role depends on his schedule. If you're a conspiracy theorist, you can speculate away about whether Chow's really passionate about Black and original superhero comedy, or whether it's those vague "creative differences" reported earlier.
Judging from the comments, Rogen's Hornet isn't too popular anyway. I was unsure about the comedic angle the project was taking, but Chow's casting was something to hang your hopes on, particularly if they reined in his slapstick style. But maybe that's precisely what the "differences" are. Maybe they just need to quit forcing this one, and let the Hornet sit for the unforeseeable future?