The .222 appeared with the Remington 722, what, 75 years ago? It rapidly earned a reputation for outstanding accuracy. It proved its excellence in benchrest competition.
When the .223 was adopted for miliary use, as the 5.56x45, it rapidly outpaced the .222 in popularity.
The .222 may have a slight edge for accuracy, its long neck, versus the .223's short neck, is an advantage.
I have a vintage heavy benchrest .222. It was built by Hart - 1A action, Hart barrel, Hart 2oz trigger. Frustrating rifle to shoot - if you see more than one hole, you know you've blown the group.