Your reason sounds logical to me. Having used all three terminals for a number of years I know that the A330 can land there. I don't think T3 can handle that size of aircraft without blocking another gate which might have been the problem being high season for CUN traffic. US could split ops to T2 if they wanted but it wouldn't be an easy or efficient operation to say the least.Something doesn't pass the smell test here.... Up until recently I had been flying CUN turns two to four times per week, and saw a variety of A330 aircraft from airlines such as Mexicana, LTU, and AirTransat. The airport has also seen larger aircraft in the past such as the L1011 and B747.
Maybe the company failed to discuss and arrange gate and parking options with the ground handling company. I do not know how many A330 capable gates are usually available in Terminal 3 where US Airways operates. Then again, there should be plenty of room over on the Terminal 1 apron for a hard stand / bussing operation. Would be interesting to find out who really dropped the ball here.....
Assuming that you are correct I wonder what would happen if a few airlines dropped a few flights to send the Mexican government a message that corruption cannot exist if they want travelers to come spend their money in Mexican resorts. At the end of the day the Mexicans need us more than we need them and the sooner they clean up their mess the better.
(BTW, for the record, I like Mexico. But the corruption needs to be addressed and sooner is better than later.)