An easy plurality of the families in Harris County are directly connected to the oil industry. That's the main reason. Also, minority turnout lags other parts of the country. The other dominant industry is healthcare which does lean left on the whole and there is a traditional urban liberal base. The county is unusually large geographically and includes a lot of areas that would be considered exurbs in Northeastern setting and vote 75% R. The city of Houston itself is probably closer to 60/40 D than 50/50. Imagine if everything within 40 miles of Atlanta were one county. It's a lot like that.
That's exactly the comparison that I thought of.
Houston probably isn't that different of an animal politically than Atlanta or Memphis, but Harris County is a whole different animal than Shelby or Fulton/DeKalb County.
