Basically GMS seems focused on rapid 2D game creation. Of course, a person can build their own or use Tiled and write their own tileengine or get something from the store or simply not use tile maps at all. In Unity as far as I know there still isn't a built-in tile map editor and engine. I'd say it's probably just being able to more naturally connect with the GMS workflows and all of the integrated tools would play a big part in that.
It could be an artistic level difference I suppose. That implies a knowledge or skill level difference and from what I've seen Unity gets more first-time game devs than probably any other game engine.
I don't think everyone using GMS are unable to handle Unity.
A person can use whatever they want to get the job done.
