Because, as dontomasso stated earlier in this thread:

"Pentangeli made an immunity deal in which he was not prosecuted for anything, and got to live under Federal protection outside of a prison in exchange for an affidavit implicating Michael in criminal activity, and he also promised to testify against Michael if necessary. This means he broke Omerta, and disgrace himself annd his family."

To have testified publicly in front of his brother no less would've caused even further disgrace. Frankie just couldn't bring himself to do it.

I used to think that Michael was threatening the life of Frankie's brother, and that's what compelled him to recant. But this is one of the times where a reason offered on the Gangster BB seemed to make more sense.