I think the Airbus order was a legacy from Continental. United are Boeing true and true...
Continental never ordered the A320. They did have a number of A300s when Gordo took over. He moved reasonably quickly to rationalize the fleet as an all Boeing fleet. In his book, From Worst to First: Behind the Scenes of Continental's Remarkable Comeback
, he mentioned that the main reasons for this were relatively poor fuel efficiency, maintenance issues, and the fact that Continental management had the planes based in one place but the spares stored at another, which exacerbated the maintenance issues. By going all Boeing, Bethune was supposedly also able to wrangle some pretty nice purchase terms from Boeing; exclusivity can confer certain "most favored customer" benefits.
Anyway, Continental did have the A300 but the A320 order was a United order dating to well before the merger and there was certainly some surprise if not shock when that order occurred, as it represented a serious penetration of the American market by Airbus. The oldest one I can find that's still flying is N403UA, which was delivered in December of 1993. I remember flying a nearly brand new one in the late nineties, DEN-SNA, my first ride on any Airbus product. Since the merger, and with so much of Continental's old management taking over the merged company, the pro-Boeing outlook has continued. Having said that, I'm sure that if Airbus offered a product that met United's needs at a competitive price, they'd seriously consider it. They have shareholders to appease, profits to earn, and a route network to maintain, after all, and blind loyalty to Boeing will only get you just so far.