Bill:
A very basic reason is that some packages require 1GB of RAM to build in finite time and there are no arm and m68k buildd with that amount of RAM.
Could you try those packages on hedges? (You can get developer access from Wookey if you need it). Hedges has 512MB real and 1.5GB swap. And unlike leisner, the netwinders, or nslu2s, it's expandable if needed.
An alternative is to use distcc+crosscc to a distccd server with 1GB of RAM.
I've done this before (rebuilt X that way, just as a test!), but it has similar concerns to the QEMU approach.
But on that point, I've never had any issues with either distcc+crossgcc--- which I've tested extensively--- or QEMU. But forcing the use of real hardware wherever possible means (a) you know for sure, and (b) you have to keep your real hardware maintained. Those seem like good things.
b.g. -- Bill Gatliff bgat@billgatliff.com