Partly because here in Washington, D.C., and in many other parts of America, the powers that be wanted to get black people out of the way. From Vox:
I’m in Southwest D.C. We have a highway here. We also had a thriving minority community decades ago, but then urban renewal came along and many families were relocated.
The PBS special “10 Towns That Changed America” explained the significance of the Southwest urban renewal. It begins around the 37:20 mark in the video below.