Not a technical reason: I've been using Django Ninja to medium/big app with lot of validation logic, and it was a delight. It was easy to start and integrates well with Django, but I'm moving away from it, not because I dislike it, I still like the approach and ergonomics. However, the project seems somewhat abandoned or at least struggling with updates (last release is in August 2024) and with lots of PRs staled. There are forks like django-shinobi https://github.com/pmdevita/django-shinobi that are moving forward and have releases
Right now I'm shifting to FastAPI + Django since much of my logic is already in Pydantic classes, and authentication is token based, so FastAPI mainly is a wrapper for my Django app. Still, it doesn't feel ideal. I would prefer to use django-shinobi, but when I first try to migrate to FastAPI I did it most of the hard stuff in a day
If you're considering options, I recommend giving django-shinobi a try. I hope it gains more traction and involves more maintainers.