Because Winning Is Most Important
Governor Schwarzenegger apparently thinks that winning is more important than actual principals. He is urging Republican candidates for President to move towards the center and capture centrist votes on things like health care and education.
Schwarzenegger won a 2nd term here California (a liberal state) by essentially becoming a liberal. He seems to have abandoned any pretense of conservative principals which should have been obvious to even the most loyal party hack when he signed SB 777 which forces the promotion of homosexuality on our public school children.
What's the point in winning if it's not so you can implement the things you say that you believe? Why even bother to beat Hillary if whoever gets elected will govern like her except he will have that 'R' label? Why debate, why campaign?
Personally, I hope the "star" GOP candidates take his advice; it can only help Ron Paul who will stick to his message of smaller government and more liberty.