My guess is detail and economy of scale. Airsoft guns commonly use the design of a really world gun either licensed or legally distanced so can be very close to reality in looks. Airsoft is also quite a popular hobby so many manufacturers are involved. You are also looking at an M4 variant which is the single most popular Airsoft gun in the world. There are literally hundreds of variants running from cheap manual springs to high pressure air tanks and costing thousands. Prop guns are more niche, they are detail orientated and don’t need to function the same way. Maybe they make a pop and eject a case from the side of the gun which is more complex to do mechanically than send a little plastic bb out the barrel. There are less people buying each type of prop gun though so it will cost comparably more per item even if it technically doesn’t do as much.