Who has heard of gentrifying?

There is a big uproar in Denver over something called “gentrifying”. That’s essentially where people start moving into a “bad” urban part of a city and upgrading it, making it attractive to the middle class. I would think those already living there would welcome such a move. It brings in new businesses, cleans up the area and makes it a better, safer place to live.
I guess I was wrong. Minorities are protesting these moves claiming it is taking away their “cultural” environments. In other words, it is changing ghettos into areas where almost anyone might want to live. Can anyone explain to me why such a move is “bad”? We constantly hear about how terrible minorities have it, living in such areas, and when people actually go in to change it, they get hostile and protest any changes.