For the same reason that the KC-46A needs them and the MRTT doesn't, no? All of the former are unable to fully utilise their underfloor cargo spaces because of the need to fit auxiliary fuel tanks. Without the need for auxiliary tanks, the MRTT is able to utilise all of its underfloor cargo space, and so doesn't need a main deck cargo floor or cargo door.
That's a bold assertion, and one that does not comport with reality.
Can the MRTT carry full 463L military pallets in either the belly or main deck? Nope.
Can the MRTT carry medevac litter stanchions in either the belly or the main deck? Nope.
Can the MRTT be reconfigured from the tanker mission, to the cargo mission, to the passenger mission, to the medevac mission in under one hour? Nope.
Can the MRTT use USAF's existing roll-on/roll-off mission kits? Nope.
The answer is "Yes" for all the above for the KC-46.
If the abilities above are important to the customer, and one aicraft can do them and the other cannot, which should the customer choose?