I think it has more to do with the nature of the crime. For example most women aren't executed because they killed their husband or someone they knew personally adn had a 'reason' to kill.

A lot of black on black crime is gang related or a smaller crime gone wrong (like lasy year a black pawn shop employee killed a black kid in my city cause the kid was trying to rob him. He got off completely).

A white man who commits murder and is given the death sentence often it appears to be because they are crazy violent sickos.

This is why it is easier for the State to kill a white man than a black man. Again, according to the numbers from the NAACP website.