Why do so-called "alternative" radio stations only play MAINSTREAM acts like Nirvana, STP, Foo's, etc?

I listen to the local alternative rock stations a lot, but I never hear any real alternative music. All I hear is the most mainstream acts that WERE alternative, LIKE 15 years ago. Why don't they just leave the mainstream stuff to the rock stations that play them all the time anyways? Are they afraid to play real alternative, new, and cutting edge quality rock that is coming out every day?