Republican primary voters like macho candidates and no one is more macho than Sarah Palin.
In a way the fashionista and mother of five evokes more toughness than any man regularly mentioned as a possible presidential candidate in the GOP. Palin is visceral, in your face, relishes combat, and won't be shamed. (Her hobbies include ice fishing, snowmobiling, running, and hunting.) These are traits usually and stereotypically associated with men - and they're especially endearing to Republicans, who like to see President Obama hit with the most force possible.
The basis of her popularity says nothing about whether she will or won't run for president or even if she could win. It does show that she's passing the Republican test of who is the toughest person on Democrats and Obama. It's a "macho primary" of sorts.
Palin goes after Obama at every opportunity and hits him hard. She doesn't use a politician's favorite tactic of saying at least one nice thing about the opponent's character before shredding a man's past, questioning his judgment, beliefs, and priorities. Palin shows as much mercy as Jack Bauer.
Not only is Palin tough on Democrats, she seems to genuinely enjoy beating them up. Crowds get behind Palin's attacks of Obama because she makes being mean fun by using humiliation, sarcasm, cutesy catchphrases, and smiles.