Few advantages I see going for ARM on Raspberry:

- machines released ten years ago are still produced/supported (the PI B+ from 2014 is still being produced)
- it's really cheap (no hardware barrier), plus already widespread (61 millions of PIs have been sold since launch)
- I guess it's the most opened ARM platform (gfx drivers are open source,...)
- the Raspberry company is opened to more OS on its platform (there is already a version of RiscOS for the PI), and Eben is fan and has fond memories of the Amiga
- It's fast enough already
- I think there is room for a light/fast multimedia OS like MorphOS on these boards
- Imagine what you could do with these GPIO pins :)

I guess it's the platform with the easiest (like free) entry fee for MorphOS. It doesn't mean it would magically work though.