AW is very public, and it is really easy to say really stupid things among the multitudes of threads you can get lost in. A lot of these people were TWFT/OPWFT/RM posters, and there's a lot of crazy stuff in those threads. I know *some* AWers have moved on because of that, for sure. I don't know if most or all of them did so because of that.
I think people also just get busy and you have to limit your time online to certain spots. Their time on AW may just dwindle to nothing and then it's gone.
I think there are some that decide that they can "move on." They've learned all they think they can from AW, now they're published, time to move on.
There are several big names that still are publishing and still contribute their time to AW. Some of the mods come immediately to mind.
But I think in some ways if you make it big like Veronica did, it gets harder to be in the same place where you want to socialize in the TWFT thread, maybe, but people may be joining it and find you and go, "OMG, I LOVE YOUR BOOK! I'M YOUR BIGGEST FAN!" I can see that being uncomfortable. (I'm reminded of being on the official Serenity (movie) forums way back when, and, when Adam Baldwin posted, he wanted to talk politics, and he'd say thanks to squeeing over his presence, but he'd expect to move on after that and talk about current events).
There may also be pressure from publishers or agents to limit their time in public forums like this.
It's going to depend on the person.