cos they own the hardware, no matter how 'obsolete', and the rights to decide who runs what OS on it. Linux is effectively hacked onto the Macs, and not all are supported.
to do it properly, IIRC, would require hardware documentation ? please feel free to correct me if I'm talking bo!!ocks...
as a side note, I would be interested to play with Morphos on my A1. or would I have to rename it Teron first ? ;-)