Based on what I read so far, and thanks to Flocks especially , above , lets turn on the speculation mode :
They seem to have "forgotten" to manually turn on the heater(s) as I do not think with the weather at the time one would have done this voluntarily. Now the system , as I understood it, should have generated 3 ECAM yellow warnings .
Maybe ( speculating) those messages were not visible because of possibly another warning , that superseded those in the priority tree, and that warning was OK on the MEL.
We do not know enough about the Russian avionic ECAM logic , but that could have added a hole in the cheese layers.