At the time of Ryan's selection (late summer), Romney was still very much on the track of trying to win the Midwest to win the election. It wasn't until September/October that he saw the writing on the wall in Ohio and Michigan and Wisconsin and started shifting more resources back to "traditional" Republican states like FL, VA and CO.
Paul Ryan was a young, conservative legislator from the Midwest who came from a humble background (unlike Romney).
FL, VA and CO were not enough to win the election without Ohio.