There are a lot of factors that go into why Democrats seem to be better at email fundraising than Republicans.
Personalities matter – it’s easier to fundraise for a rockstar (Obama) than a perceived run of the mill candidate (Romney).
Audience matters – Republicans just think and give differently than Democrats.
Issues matter – emotionally driven politics that appeal to a lowest common denominator see better success than something you have to think about to really grasp and care about (also why Democrats have cooler bumperstickers).
But the biggest issue is really the same battle that Republicans and Democrats have been fighting in inboxes, on the airwaves, in mailboxes, door to door, and face to face: