I have heard that most US retailers won't stock AO rated games but have never understood the reasoning behind it. I mean advlts Only is 18+, correct? In the UK our highest rating is also 18, yet almost all places that sell games sell all of the ratings categories.
It's an issue of public perception. The "AO" rating is considered by many to = porm. And there's a big social stigma against porm videogames.
(There's similar problems with the NC-17 movie rating. The rating agency came up with it so they could have an "advlt" rating above R, for controversial/deep/whatever stuff that was truly only for advlts, without having to use the X rating.... but it backfired. People still considered "advlts only" to mean six, and the vast majority of movie theaters and media stores refused to stock anything NC-17 rated.)
What it comes down to is that the M game rating, and the R movie rating, are the highest socially acceptable & usable ratings in the US. Anything higher than that, and no mainstream chains will touch it with a ten foot pole. So, no-one bothers with the higher ratings, except for people trying to push buttons or make six stuff. Which only reinforces the perception. :shrug: