Obscenity is often the last stop on the path to artistic bankruptcy. Wayne spent a lot of his career impressing people with abstract similes and weird punchlines. Personally I've always found him corny, but I'm in the minority: most people seemed to have thought his mixtape/early Carter stuff was dope or whatever.
Drugs and a general environment of excess/exaggeration in mainstream rap lead to this. A nikka can't just rap about eating p*ssy anymore, he's gotta rap about eating p*ssy in the most ridiculous manner possible. Example:
You so wet my tongue gonna scoop ya
Dive all in that p*ssy, scuba
My dikk head said it's deep
Gonna put my other head in and ride around in my jeep
You know, stupid shyt like that. Not only is the nikka eating p*ssy, he put his entire head inside the p*ssy so the girl is flailing about on top of him as he drives his jeep. Probably doing donuts or some shyt.
shyt is just corny, but it's the only shock value he has at this point.