Lemain: in a nutshell, there is more than one kind of ice up high. Numerous posts in the three threads covering this crash discuss the micro crystals outside of usual icing environment that can be encountered and, due to size, essentially frustrate the pitot heating that usually works just find on other kinds of ice.
AERO - Engine Power Loss in Ice Crystal Conditions
Cracking a high altitude mystery - News - NRC-CNRC
While that articles discuss is a problem with engines.
What apparently happens in some pitot tubes is the ice crystals adhere, melt, then bond to the metal, which insulates following ice crystals from the heat and leads to build up in other parts of the pitot tube, heat on or not. Some months ago an early utterance from BEA seemed to support a previous industry finding that some models of pitot tube (Goodrich in this case) are a bit better at dealing with that problem than others (Thales), but numerous posters here have pointed out that until the regulating authorities (globally) create an agreed standard, it may be tougher to require/enforce a specification. This crash is perhaps a motivating factor in moving that process forward.
See this for added info ...
A330 Pitot Tube Icing Concerns Persist | AVIATION WEEK
Europeans Require Pitot Tube Modifications for A330/A340 - >> The Cranky Flier