There are generic answers:
- Refactoring business logic costs. That's why it doesn't happen often.
- Interop between two different codebases (new Julia, old Python) costs (both at an org scale and at programming scale).
- It's a new lang. Hiring new programmers to a new meme language is very risky. You lose that programmer, your project is in trouble.
- You are underrating Python ecosystem strength.
- Python first mover advantage.
There are Julia specific reasons:
- No Julia programmers outside of MIT where as everyone has touched Python.
- Python is "simpler".
- Last I read Julia has some growing pains, like REPL boot times?