I've wondered it too. From an outsider's perspective, it seems that it should be pretty strong R. But it's a lot like Minnesota in that it has a large Scandinavian influence that makes it more progressive. Remember, Minnesota is unique in that some its most rural areas are the most D while the suburbs provide the bulk of GOP voters.
Farming also isn't as large of an industry anymore either. A lot of high tech jobs have moved in which has also contributed to its more democratic tilt.
Those elements really aren't that significant, IMO.