(creds: I was down to McMurdo as a researcher three times)

I suspect this has to do with space and weight constraints, and probably a touch of old-school procurement practices.

In the not-too-distant past, basically everything was flown to south pole station, so weight was at a premium. Powdered milk weights a lot less than UHT milk. Now they do a traverse to the pole with sleds and tractors, so weight is less of an issue, but volume might still be.

On top of that, procurement may be slow to change. If, in fact, weight is no longer a constraint, it might take years for procurement to change to include buying UHT milk.