For the same reason I voted for George H. W. Bush and not Michael Dukakis in 1988, even though all four of my grandparents were Greek immigrants.
I don't base my votes on parochial matters.
I believe in the Buckley rule, promulgated by William F. Buckley, which states: “Nominate the most conservative candidate who is electable.”
To me, that's Gingrich.
Santorum is more electable and is more conservative.