I haven't heard too many good things about either Denver or Dallas. Chicago seems like a natural choice for Boeing. It's a big city, plenty of things to do, huge white collar workforce, fairly close to NY and Washington DC, etc.
The main reason they wanted to move was traveling times and expenses. Washington is an a fairly isolated corner of the country. Boeing has industries all over the place. That's why they were considering 3 centrally located cities (Chicago, Denver, Dallas).