The selection of Palin is about energizing the base, the right-wing christian nut jobs (RWCNJs). It's classic Republican strategery - energize the wing nuts while distracting everyone from the real motivation. McCain desperately needs the votes, money and organization of the RWCNJ's and they were, up to now, unexcited about McCain to put it mildly.
So why not Huckabee or Brownback? Too obvious.
So why not someone like a Kay Baily Hutchison who has at least one thing in common with Hillary - some experience? To put it simply, she's not McCain's type. John McCain does not do old when it comes to women. He likes 'em young and pretty.
It's really that simple.