I'd like to answer that question if I may . I wasn't aware AR's are King but far more popular NOW than days gone by . My reasoning is AR10 platform would account for their increased popularity ,as it overshadows it's smaller sibling in several different aspects .
As a former Southeast Asian Vet. , I can assure you or anyone else M16A1's were NOT highly thought of ,as multitudes of problems arose with their usage .
I WON'T go into what malfunctions were most prevalent on this thread . Survivors wouldn't own one and a different mindset for a different time frame ,turned many away from AR's . PTSD in MANY cases was never diagnosed back then ,resulting in less ex Military purchasing AR's . Most wanted to leave it behind and unfortunately MANY MANY weren't able to do so .
After revision and release of M16A2 variant then the 3 and 4 series ,along with proper CLEANING products and refined idiosyncrasy's being ironed out .
Popularity based upon weight reliability function and accuracy ,KICKED STARTED the AR phenomenon . Again IMO .
I personally like the Larger .308 ,6.5CM AR 10 platform or it's equivalent DPMS style but do own a few 5.56 as well and some are #1 hole multiple round shooters . SO ACCURATE THEY ARE . I also prefer full stock furniture ,as I'm NOT fond of a putter or a waffle with a hole in it for a stock .