Because he doesn't want to go down in history as the man who thwarted "the first black president"'s agenda. Had Obama been white, Roberts would have slammed him.
Yes, it was a stroke of genius when the Democrat Party realized they had a Presidential candidate that to criticize him would be politically incorrect. If anyone criticized their homosexual Muslim Kenyan President, they would be immediately chastised for being a homophobic Islamophobic racists.