I would guess it's due to young voters and educated voters disproportionately voting for Democrats. They seem more likely to even be aware of the ability to vote early and more likely to have difficulty getting time off on voting day (more likely to work 'shit jobs'.) "It's convenient" mostly sums it up I'd say. Some strong Republican demographics (retired for instance) seem less likely to be aware of this option or find it no more convenient to vote on election day than early. edit: This is 100% postulation.
I would think that the reasons you outline are valid. I've never voted early before. I always feel like something big could be divulged last minute. For example, what if we find out Obama is really a secret Muslim socialist that was born in Kenya and wants all white people to pay a 90% tax rate? It could happen, maybe CNN will get their hands on his diary.