@OP: Rubbish.
SP was hilarious and original for the entire first season and much of the second season but somewhere between the latter half of the second season and the beginning of the third they became extremely reliant on schtick and really lost their Midas touch. Imo, the episode where they definitely pulled themselves out of the mire was Scott Tenorman Must Die (which was from early in the fifth season.) Ever since then, they've still made plenty of dud episodes but the hit-to-miss ratio is right on par with those first two golden seasons before they seemed to hit a creative brick wall.
SP:BLaU was pretty damn good though and I guess it could be for any combination of these possibilites:
1. They began writing it when they still were on that creative hot streak since, although the movie came out in summer '99, they very well could've been writing it since '97.
2. They may have been putting the lion's share of their good material into the movie while the episodes of the series that aired while they were writing the movie were getting the leftover scraps.
3. Given how busy and eager to work on new projects they were (Baseketball, Orgazmo, Team America, DVDA's music, etc.), they may have had a complacent attitude toward SP and figured an early death for it could help them avoid being pigeonholed as "Those South Park guys." (The side projects I listed were just for the sake of providing proof of their craving for branching out. I realize most of those didn't surface until later on.)