Bam was a professional skater before Jackass, made plenty of money and was well respected by the other pros.
Knoxville basically fucked everyone but himself on Jackass, he made the lion's share of the money and people like Bam and Steve-O who were responsible for some of the best moments on the show got fucked. Steve-O made the SAG minimum for his part in Jackass the movie, supposedly they promised him a big chunk in the sequal which will never happen.
Wild Boyz and Viva La Bam are opportunities for some of the other members of the Jackass show to actually make some money. Knoxville is set now and walked away.
All Knoxville did was basically take all this material that was already out on underground skate videos and bring it to the main stream.