It's the Texas syndrome. Plus a desire to become the major hub in the US, which didn't quite work. DL had a huge hub at DFW which it has now abandonned (at least 40 gates). The major failing of DFW was a lack of international flights, plus lots of delays for various reasons including weather. The airport was quite successful (thanks to its political clout) but never lived up to its ambitions.

The airport has some neat features, some of which didn't work out. For example the AA jetways were designed to be far away from the building to allow AA's MD-80s to use reverse thrust to back away from the gates (without using a tug). This turned out to be a bad idea and is no longer done (so AA uses a tug for all planes, but is stuck with those long jetways to keep the planes away from the terminal building).