I think I can sum up why you see more of AGS games than games using any other system in three words.
Big Blue Cup.
AGS has a website dedicated to promoting games made by people using the engine. They have regular competitions and other ideas to promote game creators. To put it bluntly, more people play AGS games because they know they can go to one place and hear about a load of new ones regularly (regardless of the quality)
Correct me if I'm wrong here but I've had a look and I can't find equivalent sites for either Wintermute or SLUDGE. I've also visited the Adventure Maker website today (thanks to a
thread in this very forum) None of them have the promotional power of the Big Blue Cup website.
I'm speaking as someone whose name appears in the credits of one of the
Wintermute engine's success stories. and who is a big fan of
another one. I think, for the most part the prejudice you perceive is no more than people not wanting to look too hard for new free games and knowing they can find at least mediocre new games in one place.
Oh, and Out of Order is waaaay better than The Apprentice. The uninitiated can get it
here.(Side point. A lot of sites came up in a search for this link. Maybe it isn't as obscure as you think it is)