Not sure that's exactly true. I think it's more that Dems recognize the authoritarian/elitist/hierarchical power structure that Republicans are loyal to, is one that leaves them at an unfair disadvantage. That may be due to race, sex, religion, educational status, unwillingness to compromise or be a 'team player', or any of a number of other factors. So dems side with the party that tries to appeal to a broader base of constituents rather than accept being a lower tier minion of the 'one way, our way, is the best way' nepotists.