That certainly plays into it as well but on the other hand GameMaker is not for free either and there's a huge Community there too. Of course it's a General Purpose engine so it automatically attracts more Users but yeah.
Having said all that: VS has a great track record of commercially published Games and the development of AGS is kinda stalling so people are looking for alternatives and there really is only one other big player in the Adventure Engine field which iiiiiis... you guessed it
So i think there might come a new influx of Users from AGS and hopefully this Community will grow with it
I'm certainly one who advertises VS whenever i can and the price is very reasonable for what it offers.