It's arguably the only reason this thread has as many pages as it does and no one's even attempted to answer it. The rest of the trailer is well tread ground, however.
Because we can't answer that.
I enjoy South Park, but even I am confused by their choices here and understand why people are appalled by it.
Why did they make Kanye a fish instead of depicting him as Kanye West? To avoid legal troubles? Morgan Freeman is in the game as a vendor, so I don't know why Kanye is being depicted as an actual fish
Why is the fish representing his mother look like it's in Black Face? I didn't know what "mammy" was until this thread, but my only assumption is that since they don't really know much about Kanye's mother they just made some quick carciture of a black woman to act as a surrogate for her.
The only reasoning for the joke I can see is so that it is brutally mean spirited which South Park has done in the pas or additional context or reasoning was missed because the gameplay section was fast forwarded here. There isn't really much nuance to this asides from a brutal dig at Kanye.
So yeah, the only reason no one addresses it is because we can't. We literally only can say "It's just South Park being South Park" because there is no real reasoning in this section asides from being a dig at Kanye, and the show has done racist caricatures before.
So yeah, this isn't me trying to say people shouldn't be upset by this, because it is completely justified to find fault in this depiction. But I don't think any of us have a good explanation for why this is like this asides "This is South Park".