I'm not aware of any resources explaining the "why libGDX" but here are some differences, speaking from my own experiences:
- Code oriented development, no authoring tool, no drag and drop, just you and the API, which might attracts traditional devs who prefer a pure coding approach.
- Very thin abstraction over the platform graphics layer, it just adds a few more drawing APIs over the underlying graphics API (OpenGL and WebGL). You’re free to build your own abstractions on top of the core APIs.
- Java, while might be verbose, is very stable, easy to learn and has huge ecosystem. Or you can just use Kotlin.
- Once you learn the ins and outs of the framework, it actually has a greater sense of freedom compared to Unity, Godot, Unreal, etc because those engines always force you to do things in their own opinionated ways.