It turned out for the best that Castellano's demand for money was denied by FFC and the Pentangeli character was created, if for no other reason because of the outstanding performance.

Those close to Michael were hand picked by him, namely Rocco and Neri. Tom was a brother (sort of) which explains his being held over, even though he was effectively demoted by Michael, which left Frankie as the only one who was Vito's man.

It had to be a non- Michael person to be the traitor, such as
Frankie was...don't forget he thought michael tried to kill him, he thought Michael was giving his loyalty to Roth and not his "own blood," plus he was looking at a life in prison if he didn't turn state's evidence. When he did he thought he had nothing to lose.