Why are Republicans so much stronger/Democrats weaker at the state level?

[Frodo]

Republicans have apparently decided that given the nationwide demographic trends moving against them, the House would be their best bulwark against what they perceive as a siege.  And to ensure their control of the House, they invested resources in taking over state legislatures and governors' mansions, particularly in the South and Midwest where Democrats have (of late) proven weakest, thereby controlling redistricting.