Most bands start to suck when they get popular.
Crying "sell out!" every time a band you like makes it big is obnoxious as hell, but here's why it keeps happening year after year: because when bands make it big, they usually start to suck. There's a price that comes with mass appeal and major label obligations. The fact is that casual listeners who rely on radio and iTunes' "what's hot" section generally have different taste than the people who are active music listeners.
Just look at the Billboard charts and ask yourself why none of the most popular songs in the country correlate with what you consider good music. Sales and success aren't tied to talent or quality; they're tied to accessibility. Call it music snobbery if you want, but nobody is hating on bands simply because they got popular. It's because getting popular usually comes with sacrifice, and it usally leads to an obvious, watered down version of what we liked from a band's (cringe) "early stuff."