Because nylon melts at a fairly low temperature and if there's a fire, the heat could melt the nylon. The best thing to wear on a plane is natural fabrics that are easy to move in.

Tennis shoes are pretty much required in any trip involving O'Hare anyway. I have never made it to or from a gate in that place without walking at least half a mile, usually having to go downstairs and then back upstairs, for reasons I have never quite understood. I must be on the wrong airlines.