Because he is an asshole--is the short answer. But, as Friedersdorf notes, this says more about conservatism than this one misogynistic idiot. Conservatives celebrate this man, no matter what he says. Former Veep Dick Cheney joked about waterboarding with Limbaugh, and conservative organizations give him awards.
Obama called the young woman and told her that her parents should be proud of her. I can only imagine what that meant to her. By contrast, Mitt Romney hasn't said anything, and Santorum dismissed it as Limbaugh being an entertainer.
Adult males, supposedly reared in a religiously moral context can't just call this what it is. I am embarrassed for every conservative who votes for this party--a party so cowed that they can't stand up to this man.