Gerrymandering has a lot to do with that, of course, the Pubs have been very vigorous in locking in their temporary institutional advantages. Also, 60% of governors can still represent a closely divided electorate, if, e.g., each of them won by only 3-4% or so.
Compare aggregate votes cast nationwide for Dem and Pub candidates, and I think you'll find the margin shrinks.
Come to Illinois and talk to us about GOP gerrymandering.