a) Because the Guidelines for a firmware update speak of CD-R only and do not mention DVD+-R as alternatives.
b) The DOS to be started from the disk might have drivers incapable of loading more than itself into the RAM.
So some positive confirmation based on experience or expert knowledge would definitely help.
BTW, my question has a reason: My DVD drives are having difficulties with reading CD-Rs of those types I have at home. They do not have any problem with DVDs though.