Good question, there are a few reasons - maybe not all good and maybe specific to me.
I have used PixiJS and Colyseus before for prototypes so I was familiar.
My day job is a developer, so I know how to code - but I do not know any of the game engines. So to be productive right away I opted to stick with what I know instead of trying to learn something that I find somewhat baffling. If I was going to use a game engine DragonRuby seems to be closer to my style that something like godot or defold.
It runs natively on the web, it's convenient to build downloadable electron versions but I may ultimately host the game with integrated multiplayer rather than requiring a download.