Giuliani is the frontrunner by default

The main reason that Giuliani is the frontrunner is simply by default: the other two prominent candidates, Romney and McCain, are even more unacceptable to the bases. McCain has never been trusted or accepted by them, has further alienated them with his recent stand on immigration, and worries them with his more stubborn and deluded than thou support of the Iraq War. And Romney is highly dubious, being as he is the former governor of Massachusetts, the most notoriously liberal state in the country, is constantly having to explain past statement he made to get elected in Massachusetts, and on top of that is a Mormon which makes him a heretic to virtually everyone in the religious right.