It's because gentrification doesn't solve the problems of the poor. In this country, it only serves to displace them from their homes. My being there doesn't help them. Meanwhile, they are forced to move to another neighborhood with similar or worse conditions than what they faced before.
I agree that we shouldn't polarize neighborhoods based on race, class or any -ism for that matter. I agree taht mixed income neighborhoods are the ideal. But that isn't what happens. People like me move in. We see short period of mixed income households residing there. Eventually, it becomes a middle class neighborhood. Look at Virginia-Highlands in Atlanta. That neighborhood is almost fully upper middle class now with the previous residents gone due to high property taxes. (Yes, I know Vi-Hi was once a white neighborhood. That is beside the point.)
I want there to be some sort of solution to the problems facing poor neighborhoods, but I doubt that gentrification is that solution.
P.S. I only made the Hope VI correction because the Clinton Administrationw as given the credit for it. I realize that is what the Hope VI program has become -- or was -- before the last round of funding cuts.