@lightweight – as far as I know, they are completely opensource now: https://protonmail.com/blog/protonmail-open-source/
So are the Android and iOS apps:
The problem is that it’s prohibitively difficult for a single person to host and run. The complexities of the setup and maintenance, make it hard for anyone to run it self-hosted style. Also, you’d need to customize it a lot to run it yourself.