Probably because Formula 1 is so much more partisan. Look at the way Valentino Rossi is received by the fans compared to Lewis Hamilton. Rossi is almost universally popular, to the point where the cheers of the Spanish crowd drown out the bikes when Rossi passes a Spanish rider (though they did cheer just as loudly when Pedrosa struck back). Hamilton, on the other hand, doesn't have that popularity - he's completely polarising.
As someone who vehemently does not like Hamilton, I can tell you that it has nothing to do with his talent. He's probably one of the most naturally-talented drivers the sport has ever seen. I don't like him because of his attitude. I don't like him because at every race, there is an almost exclusive focus on Hamilton. Look at Ted Kravitz on the grid yesterday, talking about Rosberg's headache - he immediately made the connection to Hamilton's start and insinuated that Rosberg was seriously compromised by it. And to be blunt, I think that the attitude of some of his fans stinks: they can't accept any criticism of him, and attack anyone who does criticise him, but they freely dish out their own criticism of other drivers. Unlike MotoGP, it's an attitude of partisanship that has created the situation. If you listen to the MotoGP commentary, they don't take sides - but they do in Formula 1. Sky's introduction video is openly prejudicial, casting Rosberg as the villain of the grid.
I've always maintained that there is so much more to a driver than their talent. We want to see them as superhuman, but we also want to see them as approachable. My personal yardstick is "could I have a beer with this driver and not talk about motorsport?" (and Jenson Button tops that list). We idolise the drivers, and a big part of that is shaped by what we value - those traits or qualities that we consider to be worthy of respect or admiration. When Valentino Rossi finishes second, he congratulates the winner. When Lewis Hamilton finishes second, he sulks. Disappointment is understandable and maybe even to be expected, but when Daniel Riccardo finishes third and smiles so broadly that you can see all eighty-four of his teeth, sulking makes you look like a sore loser. It's like James Magnussen (no relation to Kevin) scowling on the podium after finishing second at the London Olympics while the guy in third goes ballistic.
EDIT: Thanks for the merge. I was going to copy-paste into the original post, but forgot.