I think typically people don't start out thinking they'll use Numba, they start out using Python (for a multitude of valid reasons). Then eventually they want to optimize a single bottleneck at which point Numba makes a lot more sense than rewriting everything in Julia/Rust/whatever or calling some language from Python for which no convenient mechanism exists.

And when thinking about something more than optimizing a single function, but about a large codebase and the context in which you need it, Julia is rarely a compelling option.