Because they use the trigger finger to deactivate their retention device.
This isn’t a problem when everything works perfectly, but if you’re having to draw your sidearm under duress we’re already past that point.
We banned SERPA’s in 2005 after a student using one almost shot an AI in the foot. It was February and raining. Everyone was layered up in cold & wet weather gear well beyond what they were used to. Truthfully, how often to we voluntarily go to the range in SC when it’s almost freezing and raining? So, the student attempts to draw the sidearm during a transition drill and the gun gets hung up in all that clothing. Sensing that the gun won’t come out of the holster, and mistakenly attributing that to the SERPA device, he stabs his trigger finger in again and presses the exposed trigger. The round impacts the ground between the shooter and the AI standing behind him.
Was there operator error? Maybe. We could say that he should have sensed that the gun wasn’t fully seated but was actually lifted about an inch, but in the context of a transition drill where things have already gone wrong and the stress level is high, that might be asking too much in the way of fine motor skills.
Whatever, any holster that requires the use of the trigger finger to deactivate a retention device, and isn’t an agency requirement, is banned at our classes requirement.