Because nobody has done that yet. Go over to debian-devel, talk to the Debian ARM people and the Debian installer people, and you can help bring Debian to the PinePhone.

Both the Pinephone and the Pinebook Pro recently received mainline Linux kernel support with version 5.7. Debian recently added support for the Pinebook Pro as well as the RockPro64. They have daily builds available here for most of Pine 64's devices: https://d-i.debian.org/daily-images/arm64/daily/netboot/SD-c.... There's no Pinephone build yet but IMO it's only a matter of time. That being said, "Mobian" is just a DEBOS recipe with some custom software included that a user by the name of A-Wei has been maintaining. You could clone their repo and build it yourself if you'd like, it's super polished! Here's the Mobian repo: https://gitlab.com/mobian1/mobian-recipes

>As far as I can see, Mobian is maintained by Matrix.org whom I never heard of before.

I am not sure who is maintaining Mobian, but it is surely not Matrix. Matrix.org is the website of a chat protocol, where they have a support channel. Here you can see some members https://liberapay.com/mobian

So, ehm, Mobian is just an ARM build of Debian. It literally pulls from the same upstream package archives etc. Also Matrix.org is a well-regarded free software community making an open, privacy first, federated chat software. It's the sort of people you'd want if you care about free software, privacy, openness etc. I am honestly a bit shocked you haven't heard of Matrix.org - they're fairly well know in the community at this point.

Mobian is nothing to do with Matrix.org.