In the science world, not "knowing", doesn't mean they don't have good guesses that are incredibly likely to be correct. I mean, given the known variables; that it lost communication mid-flight, and then when they got communication back and got eyes on it, it had a broken rotor blade. There is really only 2 likely options, it had some sort of lock-up in the software that caused the communications loss and it crashed due to that lock-up. Or it had some sort of physical malfunction that broke the rotor blade and the resulting jolt knocked out communications.

One of those two scenarios is incredibly likely to be what happened, but they want enough data to know for sure exactly what and when and in what order. And they are lamenting that they may never get that level of clarity.