Because acetone is a far more dangerous material than either, aside from the potentially dangerous but modest "flamethrower" effect that can be achieved with a little mechanical finagling with a butane lighter. Give me a few ounces of acetone, and either matches, a lighter, or any spark source (any of a couple of dozen popular battery powered devices), and I can create for you a brief, nasty and dangerous fire likely to cause injuries and absolute panic aboard an a/c. The same combinations are likewise capable of accidentally producing similar results.

As noted by another poster and earlier suggested by me, Acetone may actually be on the TSA's prohibited list.