Sony has an open devices program where some models end up a year or so after their release. Jolla uses this program to make Sailfish releases tuned for select models.

I think it's hard to have easy open-source support by default as ARM hardware needs a model specific device tree and some of the required software might not be fully open.

Asus might not own either all software or rights to release a fully open model themselves, even if they'd like the idea.