That's usually because those wannabe hipster assholes get into it. I've liked a lot of bands before they became mainstream. Doesn't make me a hipster. Means I've got pretty good taste in music. The Black Keys are becoming really mainstream, but I won't be turning away from them any time soon. Only problem with mainstream stuff is most artists stick to what's popular and never try anything new. Look at Bieber. He's got the singing talent, but doesn't want to stray away from his shit about a fucking girl or that bible humping shit. Why? Because the preteen girls love that shit. There is no talent in that if all you're gonna do is poop out the same turd.
The problem isn't that the artists all want to do the same shit, the problem is that the record producers and executives don't care to release music that isn't mainstream because that doesn't make money. They force artists to all follow the same formula. I think Lupe Fiasco is a great example of this. His first two albums were hardly mainstream with unbelievable lyrics and production that was different from what you'd typically hear on the radio. What happens? His record company forces him to dumb it down for his third album and we get trash like Lasers. Fortunately, it seems that a lot of rap artists are able to have creative control over their albums because there are a lot of fans who want to hear lyrical albums and because a lot of rappers are on labels owned by other rappers. You'll never have to worry about Kendrick Lamar having to dumb it down because Dr. Dre isn't going to force him to change his style. In addition, rap has a huge mixtape scene so usually even if rappers go mainstream for their albums, they'll release more lyrical mixtapes. I think J. Cole is a good example of that, although I can only hope he goes back to his lyrical stuff on his next album.
Sorry if that was all lost on those of you who don't listen to rap, but the point is that producers and record executives ruin the music, not the artists themselves.