one word.
Prosperity
The old black rifle with the plastic forend and stock was nothing special to the public. Sure, you could get colt and armalite rifles, but nobody did.
With the advent of explosive growth of disposable income in the 80s and 90s, we started asking for more. Police and paramilitary organizations, as well as the military special units, began customizing and altering the basic rifle. Manufacturing techniques improved, cost of manufacture dropped, and cheap labor and parts in foreign lands along with american design made it so that a person can turn the plain old AR upper and lower into a tricked out battle rifle that even the most elite forces in the world couldn't have gotten even 40 years ago.
It's all about prosperity as a country and an individual that opened the gates to the improved technology, and allowed that new technology to be available to not only police and military units, but also the civilian market. There is a multimillion dollar market out there for add ons and alterations. everyone is fighting for a piece of it.
How else can you explain the fact that thousands of people own and actually fire barrets? One of them costs more than my buick did, brand new, in 1979 ordered from the factory. It was a skyhawk, and had a special performance package. Total price as ordered came in under 6k, iirc.