"Better" is in the eye of the beholder
With regard to the clear "superiority" of the A330 MRTT to the KC-46, that all depends on the user's priorities.
The A330MRTT is derived from the passenger version of the A330. It does not have a cargo door nor a cargo floor. The KC-46 is derived from the 767 freighter and has both a large cargo door and a main deck cargo floor. So if cargo is priority for the user, A330 loses.
Further, the A330's short nose gear gives the A330 a decided nose-down attitude when on the ground. This make loading of cargo containers and pallets problematic. That nose gear is why it took so long to make a freigher version of the A330 and why it sold so poorly. If ground cargo handling is a priority for the user, A330 loses. Again.
If the tanker is also going to be used for aeromedical evacuation, a cargo door is very important. Hauling evacuees in litters up the airstairs and then making a 90 degree turn inside the aircraft is nigh on impossible with seats installed. Reconfiguring for an aeromedical mission by removing the seats of a passenger configured aircraft is nightmarishily slow and a montrous headache. And even then the overhead luggage bins remain a huge problem. There are numerous modular kits available today for reconfiguring a KC-10, a KC-135, a C-17, a C-5 and other aircraft with a cargo door and a cargo floor to perform aeromedical evac. So if aeromedical evac is a priority for the user, the A330 loses. Again.
The A330MRTT is significantly larger than KC-46 and the KC-135. If the user has a large number of KC-135s and desires to operate the new tanker from the same bases as the old tanker, the A330 loses. Further, when supporting forward deployed forces from size contstrained forward bases, MOG (Maximum On Ground) becomes a serious issue. MOG is why the smaller C-17 can actually deliver far more cargo in a given period (throughput) than the much larger C-5. So when the operation is MOG contrained, the A330 loses. Again.
For many (most?) Air Forces MOG is a minor concern because most Air Forces operate a small number of air tankers. But when the user has a fleet of air tankers numbering in the hundreds that must support large numbers of forward deployed air assets (fighters, bombers, transports, and yes, even other tankers) the A330 loses. Again.
If fuel burn while orbiting and waiting to service receivers is a priority, the bigger airplane burns much more per hour than the smaller airplane. And so the A330 loses. Again.
And without divulging anything specific or classified, consider that the KC-46A has well over 70 (70!) MILES of additional wiring than the base 767F. The tanker mission alone most certainly does not require all that additional wiring. There are other "things" being installed in those airframes that have nothing to do with the tanker mission. Airbus simply could not do these other "things". And those "things" were yet another high priority for the KC-46's user. 'nuff said.