Publicly revealing information about cancelled games is one of the best and also the most painful phenomena within the industry.
It is always intriguing to see what kinds of strange and insane game concepts people come up with. It is sort of like gaming archaeology. The problems begin when the cancelled game seems genuinely great, and the industry seems to be lesser for it not ever being launched. Sometimes these games spur fans and modders to revive the project. Sometimes the massive popular outcry resurrects them. Sometimes the community deems a game was better off buried and forgotten. Some cancelled games are averted disasters, while others are missed opportunities.

We have no idea what the hell this one was.
If you think about it, this is another case of the "obvious-choice-but-never-happened" syndrome. What do GTA and South Park have in common? Well, several things. Violence, profanity, sexual themes, social satire, dark humor and more violence. What else do the two have in common? Obscene amounts of fans! Considering how massively popular both franchises are, a mixture of them, even of not an official cross-over, would probably destabilize the entertainment industry. That alone might be why this game got canned.