Well considering the number of air forces that don't have a nationalistic interest in either manufacturer that have selected new strategic tankers in the last decade or so have pretty much all selected the Airbus. Also in spite of the USs traditional preference for a local product and it was only after political interference (not to mention a rewriting of the specification to specifically favour the 767 and disregard the strengths of the A330) that the original decision in favour of the KC-30 was overturned.
The other factor is how many commercial airlines are buying 767s these days? The A330 is still selling and so is the 777, it screams to the cynical (such as myself) that the only reason the 767 was ever considered was a politically motivated form of economic stimulus, specifically targeted to keep the 767 line going following, first the downturn in airliner sales following 9/11, then the GFC.
It was rumoured that what the USAF was actually after, before politics and industrial life support took precedence, was a smaller faster, tactical tanker, that would also go on to replace the KC-135 / 707 airframe in most if not all of its various specialist roles such as ELINT, AEW&C, specialist ISR etc. the sort of missions the 737 is falling into now, or the -135 is being life extended for.