They have tried this extensively. Inner city block grants, subsidizing business, etc.
The idea that you can 'fix' another person's neighborhood is equally as racist as the policies that caused blacks to concentrate in those neighborhoods in the first place (they were chased out of the countryside in the KKK era and left with few alternatives to the city). What if the government started offering grants to 'fix' your neighborhood? How demeaning that is, and how absurd! As though subsidized bike shops and coffee shops are going to pressure crack dealers to go away.
As for the police patrols and condemned properties, do you think that there is a functioning local government in these areas? Look at DC and Detroit! The local city government is so corrupt that they can't even provide basic services, or basic levels of non corrupt policing, much less an effective attempt at crime reduction. On my way to work in DC a few years ago I would see the same police officer asleep under the same overpass every single day. This was right next to the highway, so maybe 2000 people would see him every morning, and yet he would sleep away, sometimes with his face pressed on the drivers side window. Do you think a bagel shop can fix that!?