black tack is usually more readily available and cheaper. there’s also the issue of quickly getting replacement parts during a show - it’s easier to do so with black tack. probably just run to one of the tack concessions right at the show. it’s easier to borrow stuff, too, if something is lost, stolen or forgotten at a show.
i was disappointed actually when black tack became the fashion. a lot of black dyes really stain white pants, and they often take on a greyish or greenish, or just washed out tone with time. brown tack tends to just deepen and look more classy, and the stains seem to be easier to get out of pants.