Just because something is open-sourced, it doesn't guarantee it is going to be updated and maintained.
When the developers dropped Godot and open-sourced it, the only reason people jumped on the wagon was simply because of the fact, at the time Unity, Shiva and Unreal was 3D only and not available for Linux and GameMaker was not free while Godot dropped out of the blue like a super hero just dropped from the sky that did support both 2D and 3D and also Windows and Linux and it was free to everybody, no strings attached. So it was the perfect free alternative especially to Linux users which I was using at the time and this is the no1 reason why I remember so well and why the release of Godot was so epic.
But for example when the Xenko game engines was dropped and open-sourced, because of the AAA 3D graphics, there was some whispering "unity killer, unreal alternative" but people didn't jumped because it was Windows only and by this time, Unity did support 2D and Linux and we also had Godot, Defold, CocosCreator, even GameMaker Studio had a free version around this time. So people didn't care that much. Being open-source and being free is not a big deal anymore and certainly no guarantee the project going to be maintained so people choose to go with more popular options instead. There is plenty to choose from and Xenko (now Stride) is still not very popular. Still 3D only, still Windows only, still have no terrain and the developers has no intention to create a new cross-platform IDE. When asked the answer is straight NO despite the fact there is plenty cross-platform GUI libs for C# and .NET and they had years to come up with something at least some plans but nothing. The answer is straight NO. (period)
Around the time of Monkey X I was way too focused on visual game makers like GDevelop, I honestly can't say why it failed but I can imagine something similar happened. Because it was dropped, people jumped from Monkey to XNA or MonoGame or LibGDX or AppGameKit maybe and they didn't looked back.