Because it's a fork

It's a fork of Django. It says clearly on the docs.

It's a fork of Django.

Third sentence on the page.

Third sentence.

I usually click through to the repo, and it isn't in the README for some reason. I don't blame GP for missing it. https://github.com/dropseed/plain

Of course, GP would've noticed it's like Django on the web page. The screenshot containing Django-like example code is above the fold, though - the Django mention is below the fold.