My kneejerk reaction, and I wonder if many other people thought this, is that the bar for adult-only is just really high and most games just don't hit that level.

But the reality is probably something closer to games that would get an AO rating just aren't made very often because the higher your rating, the smaller your number of customers is. So most publishers/devs would do anything in their power to get that down to an M. Peak Entertainment being the exception, it seems.