The MacBooks have a plastic part on the side where the WiFi antenna is. That would mean a new PCB layout and new moulds... The cost of this is too much.Well, an aluminium body would cost at least 60,000 USD to make, and then about 10 - 15 USD per case.
So if 1000 Pandora users would want such a body, it would cost about 80 USD more... and I have no idea how WiFi would work.