Variety of factors:
One: he's ubiquitous with the sport. Which makes his target the biggest. Which is annoying because personally he's a douche. I'd say Travis Rice is probably number two popularity wise (correct me if I'm wrong) but people still love him (see below as why).
Two: He's a whore, massively so. Shaun White scooters? Get the fuck out. Shaun White gum? Target collection? Fucking really dude?
Three: He's anti social.
Four: He's a bitch, he declined SuperNatural last year. Fucking why? It's the biggest and best riders on a huge one of a kind course. Why would you decline that?
Five: Just jealousy, for 1-4 above..he's still rich and to some people that means he's a sellout.