There's lots of good reasons. Most notably the autodiff in Julia is unreliable -- there's been quite the litany of packages trying/failing to do this robustly and efficiently. (This issue pretty much kills its possibilities straight from the get-go.)

The fact that the ecosystem is largely created by academics is also a serious issue. Code quality is very low / things silently break / etc. This is partly an issue with Julia itself; the language offers few tools to verify the correctness of your code. (E.g. interfaces etc.)

Julia vs XYZ is an old topic. And yet this keeps coming up! There's quite a lot of blog posts on the topic. (E.g. here's mine: https://kidger.site/thoughts/jax-vs-julia/)