Strange, I read a lengthy and well annotated description in Wikipedia and there is no mention of gerrymandering or of slaves voting or not.  The only reference I saw to restricting voting rights is the applicable penalty under the 14th Amendment of 1866 (at which time slavery was abolished).  

Indeed, the purpose of the system as described elsewhere is to ensure regional balance; that is, to prevent one candidate with a large majority on one jurisdiction to overwhelm the vote.  This goes back to the beginning, when smaller states were wary of giving up "state's rights" to other states with higher populations.  Seems reasonable.    

Simply put, the Americans value regional balance over one state having disproportionate power.  That's also why the Senate representation is based on states, not population.

Without California, Clinton would not have won the popular vote, nor the most states.  The system seems to have worked as it was intended.