I remained loyal to Delta even when I was in Houston for five years. The main reason? Operational reliability. Secondary reason was IRROPS handling. I switched to United for the first couple of months I lived in Houston, and 75% of my flights were significantly delayed. I figured out I'd get to my destination sooner by connecting on Delta than by flying nonstop on United. Now that I'm in Boise, United is probably slightly more convenient but Delta has enough options that sticking with them is an easy decision.
Several years ago my company still had American as its preferred carrier. A year and a half of flying with them left me the impression that their motto was "Don't burden the customer with useful information." There's nothing like sitting at the gate 30 minutes after scheduled departure with no agent in site, no plane in site, and the information board showing the flight on time.
I'm not a fan of Delta's many, many devaluations for medallion flyers over the last few years, but the way the airline industry works today, United and American are sure to match those devaluations in short order. At the end of the day it comes down to the fact that most of the time, Delta will get me to where i'm going as scheduled.