The inevitable
Palin's complete stupidity is almost certainly the reason she'll win. Like watching anyone make an idiot of themselves very, very publicly, it has a certain fascination value; you cringe, but you can't help yourself watching out of morbid curiosity, which will generate her a lot of headlines, column inches and airtime her rivals won't get . Much of it may be utterly dreadful and even scary, but she's going to be so ubiquitous everyone will feel they know her, love or hate. This being the age of the amateur, a lot will vote for her because she's "just like us" - a sort of bizarre extension of reality TV in which you get to vicariously see yourself as President through the vehicle of someone identifiably as ignorant, prejudiced and shallow as you are.
Then there'll be the more educated on the right, who'll vote for her because she comes out with the fantasy, lunatic stuff that appeals to the visceral instincts, but which people know in their hearts is a bad, bad idea - things the McCains of the world are too adult and responsible to say.
She'll get plenty of big money backing, because the money will recognise that she'll be thoroughly pliant to their cause with little thought to the consequences - an empty vessel waiting to be filled and that will buy her even more media time.
The Democrats hate her, but they'll talk about her endlessly and the constant derision and negativity will push any fence sitters to the right out of sheer spite - my enemy's enemy etc.
US elections are no longer about policy, reality or common sense - if they ever were. Americans just cannot live with the reality that their star is waning and the New American will turn out to be no such thing. Palin offers a fantasy vision of an America restored to greatness in terms that are a closer match for 19th century frontier rhetoric that 21st century pragmatism, and people will bite because it will make them feel better about their declining nation, if only till the day after the count is done.
Ultimately she will win because she is folksy, venal and vacuous - not in spite of those qualities.