I can only give you a fairly logical explanation for this. Take, for example, Northwest, which began during the 1920's in Minneapolis carrying the US Mail. Over time, they began to branch out to cities that were farther away (I believe the first place they went to was Fargo) and eventually had a fairly substantial route system for passengers and airmail. They also began to place emphasis on different cities that were in different areas that did not already have a fairly good sized airline present. In Minneapolis, one of Northwest's big competitors was North Central Airlines, which I believe also had routes to and from Detroit. Anyway, Northwest started entering the south and encountered Southern Airways, which I believe was based out of Memphis. Northwest also started flying into the southwestern United States and ran into Hughes Airwest, which was fairly big in that area. North Central and Southern merged in the 70's to form Republic Airlines with hubs in Minneapolis, Detroit, (Milwaukee?) and Memphis. They were, essentially, flying the same routes as each other. Republic increased its route system by buying out Hughes Airwest around 1980. Republic began to be pushed out of the southwest by the growing success of airlines like PSA; however, in 1986, Northwest (Orient) bought out Republic Airlines (for $884 million) and managed to gain almost complete control of Detroit, Memphis and Minneapolis and to re-enter the southwest.
Incidentally, the State of Minnesota rescued Norhtwest Airlines, one of its largest employers, from bankruptcy not long after the merger with Republic, and stated in the paperwork that Northwest had to keep its world headquarters in Minneapolis until the debt is paid off. Prior to the merger with Republic, Northwest was a profitable airline that was able to pay CASH for all of the airplanes that were ordered.
Aviation is proof that, given the will, we have the capacity to achieve the impossible.