The really ironic thing is that the government MADE the 5.56/.223 the most popular caliber, along with convincing millions of people to buy AR 15s.
They did this by trying to ban the gun, and stupidly, failing.
Forbidden fruit, people.
The AR 15 and the .223 hit the commercial market in the early 60s (round 64 or so I think), and did not set the world on fire. Sales were low, there was not a lot of interest at the time.
The rifle cost more than a decent deer rifle, and fired a round that wasn't legal for deer in over half the country. There wasn't a big market for any military look alikes, even when chambered in deer class rounds. The FAL was also on the market then, and sales were...lackluster. Again, cost was a major factor, along with the military styling which didn't appeal to most of the market in those days.
By the 80s, the redesign of the AR made it much more accurate, and modular accessories were being made, in ever increasing numbers, and interest was picking up. By the 90s, the Govt was demonizing the gun and enacted a partial ban with the 94 AWB law.
THAT got people VERY interested in having one (or more) and when that law sunset in 2004 LOTS of people who never wanted one before went out and bought them, and people have kept on buying the gun the Govt didn't want you to have in large numbers.
EVERY law increasing restriction on the AR in one state or another upped the AR sales in all the other states, and it continues to this day.