What causes regional price-to-rent ratio differences?

[Mike Dymski]

What social or economic conditions cause price-to-rent ratios to be lower in the mid-west and certain pockets of the country versus other areas?

Not looking to debate the merits of investing in one market versus the other or cash flow versus appreciation...just looking to understand better what causes the ratio to be lower in those markets.  I have my thoughts but don't want to bias the discussion.