Superman should be a more relatable character than Batman, he was raised on a farm by good, hard working parents, his dad died and all that shite, he's had to go to the big city and make his own way in the world, but he's not Human, can outrun a bullet, can catch a train in his hand and is nigh on invincible, plus he stands in front of us all with this moral superiority, flawless character and goofy costume and you can see why people sour on him.
Of course, that doesn't explain the rise in popularity of Marvel's Thunder God, I believe Thor has grossed more than any Hulk film so far, does a hammer and a bit of arrogance make that much difference or is it because we haven't seen films about him since the 80's?
I've never thought of America as a Country that roots for the underdog, certainly not in the way that England is famous for. So I think if Superman was a cigar chomping Wolverine we wouldn't have this issue, personally I think Wolverine can go fuck himself, now there is an overplayed, dull, shit stereotype of a character. Still, he's an anti-hero, but even anti-hero's are fizzling out now, gone are the days of Stone Cold, as a species we find anti-villains more interesting, because they're even more bad ass; Dexter, The Sopranos, Breaking Bad, Oz, even those more cartoonish films like Despicable Me and that Will Ferrell animated film.
In short, Superman isn't bad ass enough, maybe if he had a trident, a shotgun and constantly swigged out of a whiskey flask.