Because it's bad.
Exactly, Bob Marley isn't overrated, you just don't like reggae. But Bob Marley is beyond music. He's in the class with Presley, Morrison, and Robert Johnson. Calling his music overrated is missing the point. I don't love the guy, I'm just pointing it out.
Reggae is ok. I like ska better, and I don't like ska that much. But it is impressive that an extremely culture based style, from a small and in some ways insignificant area became world recognized and appreciated, mostly because of Bob Marley. I can respect that.