I keep wondering the same. It's a significant speed-up in most cases and equally easy to (apt) install

For public projects I default the shebang to use `env python3` but with a comment on the next line that people can use if they have pypy. People seem to rarely have it installed but they always have Python3 (often already shipped with the OS, but otherwise manually installed). I don't get it. Just a popularity / brand awareness thing I guess?