In a phrase - NDs.
Primary reason I have seen is that the index finger "falls naturally" into the trigger guard and onto the trigger once the retention button is pushed. Thus the gun is being brought up - in their opinion - with a finger on the trigger and is therefore dangerous.
Personally, utter nonsense, I am wearing one right now, and when I practice the draw, the finger has never yet ever entered the trigger guard. In fact, it is always along side the slide where it should be.
I believe you can get the Level 1 Serpas and use them in the training classes, as they do not have any retention mechanism. Only the Level 2 and 3 have the retention system.