I can't remember where I heard this, so don't quote me, but I've heard it partially has to do with graphics kexts in some or all of those machines being 32-bit and a reluctance from Apple and the manufacturers (Intel, AMD, Nvidia) to update them. That said, I'm not sure how that would rule out the Mac Pro, then, because the chips I heard cited were the old Intel chips (950, x3100) and AMD chips (X1600).

Again, don't quote me!

edit: this assumes Apple is moving to 100% 64-bit.