The show started in 1997, and this movie came out in 1999. Beavis and Butt-head didn't get their big screen debut until five or six years into their show. On top of that, Matt Stone and Trey Parker don't seem to eager to have a second South Park movie after all this time.
They did the right thing and made a movie right when SP was at it's most controversial and making headlines. The infamy helped the movie and the show at the same time.
I think the reason there is no sequel is due to the fact they don't write the same way anymore. The newer SP episodes are just terrible and are more fan service than having a point as they used to. Compared to the earlier stuff that actually had something worth listening to.
Jesus died for our sins. As he's already dead...sin away.
The film was made early in the show's run because there was a great deal of buzz around the show in the beginning. The show has remained popular throughout its run, but it's not at the level it was at during its first two years.
Don't forget that the Simpsons didn't get their movie released until 2007, and their show debuted like what 1957, okay okay it was much later then that, but it just seems like they have been on forever. The Simpsons made their debut in 1989 and are still on the air. It sounded like it was a big hit the first couple of years, and maybe they aren't intrested in another movie, but then again since their movies have come out, I don't think Beavis and Butthead and The Simpons have or ever will get a sequel. Actually Beavis and Butthead do America came out in 1996, and the show made its debut in 1993, since I have to assume that you are not a very big fan of the Series. And as far as I know of Family Guy has never had a movie, or even attemted to do a live action movie of the series.
Well Family guy has done the Star Wars spin off movies, don't think that really counts tbh though.
The stick of truth is out in a couple months that's going to be comedic genius! I really want Trey and Matt to make another movie but I'm hoping the game is going to make up for that!