Because that's what his character is. He is designed, and known entirely, for a very specific look.
Tall, skinny, white (or asian as he is really similar to Samurai Jack), and a professor in a white labcoat.
If they kept the professor white but fat as fuck, it'd still be offputting because that's not "Professor Utonium", it's someone else.
If the Professor was mexican, female, handicapped, blonde, bald, a midget or literally anything OTHER than what he was in the cartoon, it'd be off-putting as fuck.
Just like if the Gangrene Gang was not green at all, or if Mojo Jojo wasn't a monkey but a human, or if Ms. Bellum was a sassy fat black lady, or the Mayor was a ripped young playboy stud, or Ms. Keane being a male, or, fuck, even Fuzzy Lumpkins was human. No one would fucking like it, and everyone would be talking shit about it.
Most of /co/ don't hate that he's black, they hate that he's not Professor Utonium.
And also that the show sounds like absolute shit.
The nigger jokes really are just easy picking. I mean, fucking hell, >>121963812
you can't tell me you didn't laugh at this. Or you have no soul.