Why don't felons carry in a holster?
Well, for one thing they would have to go find/acquire a holster that fits the firearm they are carrying at that particular time. That is probably seen as an unnecessary hassle and, potentially, even something which increases risk/exposure.
It's probably easier to ditch the gun than the holster + gun.
Most holsters are designed to be worn on a belt.
The only thing necessary for evil to triumph is for good men to do nothing.