The reason that they are popular is that once you have one in your hands and shoot it, you want one. The Army has never made a more handy light and powerful weapon.
The reason that they are $800-1200 is because part swappers are addicted to carbines and can't pass up anything that they perceive as rare. They may be dead wrong or taken to the cleaners by humpers, but they will still whip out that card and buy it. Part swappers are their own worst enemies, and when it reaches the point , which it has, that they find a pig in a poke and suspect it, but still say " I like it anyway", they deserve what they get. Because that pig in a poke will belong to some other poor sucker before too long, at $1200.