If I had to take a guess it would be relating to the outstanding AoA integrity issue that Patrick Ky referred to in his EASA presentation "the four conditions that must be met before return to service". Therefore assume that EASA is waiting on Boeing's plan to get a 3rd or equivalent AoA sensor input into the system architecture.
I imagine that hell would have had to have frozen over before Boeing will commit to it. Hope they prove me wrong.