After Continental / United Merge, United Airlines cut seveal routes to Asia and Europe, from Houston. If you see stats of Houston Airport, traffic didn't really increase since 2012. A year a growth, the year after a big decreased. https://en.wikipedia.org/wiki/George_Bu ... al_Airport
United Airlines has expanded t San Fransico and Chicago hubs. If you see, both airports have gained many new routes to Europe and Asia.
After a 9hrs flight, to Chicago, it will take you a little bit over 2hrs to reach, your destination, it's probably better via EWR, where it will take you 4h30 to reach Denver, after a 8h30 flight, from Amsterdam. Same thing, if you fly from USA to Amsterdam. 2hrs to reach Chicago, from Denver. If you can reduce transit time and the flight time, on some parts of routing, it's a good thing.
Same thing with San Fransico. The UA hub at SFO, provides lots connecting on the USA West Coast, from Alaska, to California, Hawaï, in Arizona, and Mexico. You arrive from Europe, after a 11hrs flight, then you fly to Seattle (only 2hrs). Via New York, from Europe (7hrs-8hrs of flight), then from EWR, it takes you 5h to reach Seattle.
With the merger between USA carriers, carriers were able to add more hubs (UA (Houston, Newark), DL (Detroit, Minneapolis) and AA (Philadelphia, Charlotte,...)) or expand their current hub. The goal is to have the highest numbers of passengers, using their hubs. Why not to use the full capacity of Chicago or San Fransisco, for UA ? There are not only New York or Washington in USA.