At one time, and psibly still in some areas, the gas was corrosive to the galvanized coating and it would disintegrate on the inside of the pipe and the residue would plug pilot lights and orifices. The dirt/drip leg was supposed to catch it, but some still go through. Now, however, at least in the areas I have worked, black pipe can only be used indoors. Galvanized pipe has to be installed if it is subject to weather conditions.