Why would I choose yet-another-build-tool if Maven works, is stable and backwards compatible? Even worse, why I would choose Java as my language to run a build? The only benefit is to avoid learning a tool that isn't Java, there's nothing else to it.

For some Java build tool to actually gain traction it'd need to be better than Maven in every way, that's pretty hard to accomplish and I'd say that starting a tool that runs code already goes against the best practices for infrastructure code: convention-over-configuration, declarative.