One possible reason that Tom Ridge decided not to run for U.S. Senate is that he didn't believe the GOP polling that was done showing he would trounce both Pat Toomey (R) in the Republican primary and Sen. Arlen Specter (D-PA) in the general election.
In fact, a new DailyKos/Research 2000 poll to be released later today shows Ridge losing to Toomey in the Republican primary, 41% to 33%.
Update: The poll results have been posted. In general election match ups, any of three Democrats tested beat Toomey.