Earlier quoted context omitted.
That's a very bold statement, what has Godot done to warrant it? It's not even the most advanced open source 3D engine right now, considering O3DE (a fork of CryEngine) is open and backed by a fair few big players.
Usability. For artists and non professional programmers, those options are fairly inaccessible.
FWIW, I don't think cornering the non-professional developer market will lead to market dominance. GameMaker is even simpler, and even if it were open source, hobby developers will not be competing technically against thousands of full time, professional engineers working on an engine.