Because now that it is apparent that he can win something he will face scrutiny now that will expose his liberal policies in immigration, government, and taxes.
What do you think has been going on for the past month? The "new" media has thrown everything they have at Huckabee and it didn't make a difference in Iowa. And, most of the south is similar to Iowa.
The other problem with your premise is that McCain, Giuliani and Romney have more liberal baggage in their records.