As we all know the force is all about balance. Given the fact that black there were no black Jedi on the council before him they decided to take a basic step down the road to balance in the force. The members were not too worried about letting them join, given the fact that they only had 3/5ths of a vote and would be unable to affect the politics of the Council.

Anther reason for the Black Jedi's apparent absence from the council was a growing fear that they would be easily seduced to the dark side. These fears still exist today.