Well, aside from the reasons that have been mentioned:
You have to factor in the penetration of the weapon platform, especially since the first AWB was lifted and then the fear of a second one with The Great Moron being elected.
People that shoot targets for fun and people who hunt the whistle pigs, like our own Son of the Prairie cpttango30, are snapping them up left and right because the weapon is effective, accurate and you can shoot it all day without it overheating.
Couple that with a Challenge that was set forth by Col. Jeff Cooper to ANY semi auto rifle builder to make a 20/20/20 rifle.
In under 20 seconds.
Into a target less than 20 inches in diameter at 1,000 yards.
Dave Lauck was the first AR builder to build a rifle that completed this challenged and raised the bar as to what TRUE AR accuracy is all about.
The civilian AR platform is no longer about the high rate of fire, lightweight, jungle shredding, hand held machine gun that was introduced into Vietnam.
The new AR's are about precision and accuracy and having the ability to engage multiple targets, at multiple ranges and do so without putting the operator in danger.