That's because the MSM has the goods on Rudy, and they will reveal them once he gets the nomination.
You're correct that the dirt will really hit the fan when Guiliani's the nominee. Remember how it was conveniently leaked that Bush had been stopped for drunk driving? Remember how his poll numbers dropped? And this was for something that happened decades before. Guiliani's transgressions are fairly recent, aren't they? Like a few years ago?