Romney is a traditional politician: he blows with the wind. Ten years ago fewer people were sensitive to that but today, it's different. We're more polarized, striving for more philosophical political purity and less tolerant of wavering/pandering. Not the entire electorate, of course, but a lot more than would have been counted 10 years ago (and this is true on both sides, for good or ill).
Romney's faith simply isn't much of an issue for issue voters. It is an issue for emotional voters (people who vote based on personal likes/dislikes as opposed to positions).
Having grown up in LDS culture because of my family, I can heartily agree that the LDS have some very non-Christian notions of theology and doctrine. That said, most of them 'walk the walk' in terms of behavior and personal ethics.
For people interested in this sort of thing, the LDS doesn't meet the criteria for a "cult" as that term is currently used to describe a movement lead by a pivotal charismatic leader who exerts direct control on his or her followers. The LDS come in all flavors including jack Mormons with the bulk of the flock not even holding a Temple recommend (and not caring, either).