Can I jump in and make a loose comparison to politicians?
I think it's similar to having a wonderful person with an interest in politics do well in their area. Although they still might be wonderful people they're going to do have to do the things that will get them votes or support, even if those things disappoint the people who originally backed them up. it's very rare to find a person who can please both their first loyal supporters and the supporters they've gained along the way, as often both groups have different ideas of what is right. Soon enough we startup see these people as greedy and careless.
As soon as bands see the subtle hints of becoming more popular I think they reach for those opportunities a lot of the time, not only will it mean more money but also a lot more tours and such. However it's hard to please both the loyal fans and the new fans. Often the new fans determine the way the band goes, which is a bit disappointing, and so the die-hard fans might come to see the band as sell-outs.
This, of course, is merely my humble opinion, but I think it's another interesting topic. I also find it funny how things can work the other way. Although band might not change their style, die-hard fans might turn away as the group becomes more popular because with this popularity the band becomes 'mainstream' which I think a lot of people don't like to be seen as.
Like I said, interesting topic