Some people prefer stability over software availability. Arch leaves much to be desired given it's lack of tooling that's been available for years for other distributions. It's an hobbyist project all along.

I believe postmarketOS team closely collaborates with Alpine. It cannot be said about Manjaro, neither in relation to Arch or Arch ARM.

we definetly try to upstream as much to Alpine as possible. Also postmarketOS basically maintains KDE in Alpine

Manjaro is downstream from Arch.

That's not my point. Being a downstream does not imply collaboration.

It's not true. GNOME Software still ships Snap plugin. It's Canonical that decided to fork gnome-software and provide it under different name without competition plugins enabled.