Artie and the show's producers wanted them to go to the Super Bowl and try to dupe some unknown player into saying something "controversial" about gays thus generating buzz about the show. Nick wasn't down to do something cheap like that so he left. Also, they didn't like that Nick wouldn't fall in line with media's liberal POV. Also, Artie has tremendous drug problem that caused tension on the show. Also, only so many times Nick could sit through one of Artie's six stories that he constantly recycles.
This coming from someone who watches every Artie Lange Show, but hates the show and Artie with a passion.