[shadowgovt]

If I understood the livestream correctly, it's because they were able to (maintain|quickly establish) lock to the MRO after touch-down and zip a couple of images up through the "bent-pipe" UHF-to-high-power relay into the Deep Space Network.

[GeorgeTirebiter]

It would seem they had MRO in position to snap a few photos of the landing / descent as well as do the relay.