To Phoenix's disadvantage, it was long considered a tourist (winter) resort destination. The population swelled in the winter, and dropped precipitously in the summertime as all the "snowbirds" went back home again to escape those hellish temperatures.
While the population has grown by leaps and bounds in the last 20 years, there still really are no large headquarters of businesses in Phoenix to snap up higher-fare tix. There are some exeptions (Dial Corp. in Scottsdale comes to mind), but by-and-large there are no "big fish" in Phoenix, even with a population of nearly 4 million people in the Valley of the Sun.
In addition, Phoenix is awfully close to Los Angeles, which makes it easier for airlines to code-share thru LAX
Where it is coming into its own lately, is with HP
Mexico/Canada flights. Might be interesting to see where they spread their wings next....possibly more Central American destinations to capture immigrant travel from all over the West?