It seems at the moment it is very hard to enlarge the communites of "alternative" game engines.
1. Visionaire costs money. The willingness to pay for software is decreasing in the "mobile phone generation" of developers. There are some free game engines, for example Unity.
2. At the moment the popularity of Unity is hard to combat. Alternative game engines (like Godot, Visionaire or GameMaker) are relatively unknown.
3. Graphic Adventures is a niche genre
I am not sure what is the best way to lure new customers...
I wouldn't say Game Maker Studio is relatively unknown. Pretty much anyone looking into 2D game engines will have probably stumbled into & tried using it at some point. They recently released Game Maker Studio 2, which looks quite nice. Game Maker tends to be a bit on the pricey side - in regards to the export modules.
I'm not sure what the fuss is over unity. Nearly every unity game I've played so far has been horribly optimized. Whether that is the fault of the developers or just something to do with the engine, I don't know.
UDK (Unreal 4) is also free & as far as I'm aware can also do both 2D & 3D. Not sure how hard it is to use/learn though as I've not tried using it. I think it requires quite a bit of hdd space for all the libraries & stuff that it comes with.
As you say, adventure games are a niche, so it's not exactly easy to entice people unfamiliar with the point & click genre to that. Something that would really help is if the mobile & tablet porting was made easier to develop & export as I could easily see Visionaire being used for Visual Novels & Hidden Object Games, which are 2 very popular type of games you see a lot of people purchasing - personally I'm not a massive fan of either, but they are pretty straight forward to develop & developers can easily make money out of them providing the story/artwork is nice.