From my research the problem is not with the wire itself but at the connections. Environmental changes can change the physical properties of the aluminum wire causing the connection to loosen, resistance to go up, heat to be generated, and then ignition.
See:
Aluminum - Household Wiring
I'm not advocating the use of any kind of wire. I'm just an advocate for the horse before the carriage. If FIRST wants only copper wire to be used then there should be a rule for it. The Q/A system does not create rules, it only clarifies them. Manuals and team updates create and modify rules. I understand that in years past there has been a rule that limits teams to only use copper wire. However, we all know that previous years rules don't apply to the current year.