I like Foo Fighters a lot, too, but if you listen to their earlier stuff you'll see it's a lot like Nirvana.
Why the change? Well, for one, Dave changed, so naturally so did his music. Another reason is probably the simple fact that he didn't want to be accused of just continuing what Nirvana started. He wanted to have his own claim to fame. To be recognized as an individual rather than "the drummer from Nirvana".
