There was a story floating around that the Gambino's couldn't get to Roy directly so they went to Casso since he knew Roy's crew. Casso told them that if they carried out the hit, he would take them in and make them. The catch to the story was that if they didn't they would be killed anyway so they really had no choice. Add to that the fact that Roy was their only connection so once he was gone they would be on an island.

Ive never heard proof that they were both made or that the story above is 100% accurate but both points are widely accepted and I've never seen them disputed either so I assume they are true.