Mainly because the majority of airsoft players didn't play when gas rifles were the norm, and they don't know about them.
Quantities aside is a fuckin' cop out, Christian. That's like saying ask yourself, why do people pick Volvos over Lambos? Well, that's because Volvos are cheaper, easier to come by, require less work, and can be serviced by many more people. However the case may be...Lambos sound cooler, go faster, handle better, and get you laid more than the Volvo would.
Classic rifles are worth a lot. I have more in JAC AUG mags than some people have in their guns...and I only have 5.