[BTinSF]

^^^2 decades ago, United had a major maintenance hub in San Francisco. At that time, their "history" here was as strong as with Chicago but they decided to consolidate and chose Chicago as the place to do it.

As Wikipedia says: "United's early route system, formed by connecting air mail routes, operated north-and-south along the West Coast, and east-to-west along a transcontinental route from San Francisco to the Midwest and Mid-Atlantic states via Denver, Colorado. The early interconnections made at San Francisco and Denver during this era became the basis of major United hubs in these cities, and still exist today."

They didn't start out in Chicago and I'm not sure when they moved their HQ there. But the roots aren't as deep as some think.