Why isn’t c++ a first class citizen in a game engine written in c++?
Maintaining C++ support is expensive for an engine, just like maintaining any language binding. IMO, they should optimize for the most approachable/developer ergonomic language, which C++ definitely is not.
That said, I strongly disagree with their decision to simply build their own language. I think there are many languages that already exist that make better candidates for game development either because they are well known, well supported, or easily portable.