The really important question is, "why is it not referred to as a pair of bras?"
Everything else used in conjunction with a paired body part is called a pair, even if it is a single piece. eg A pair of glasses, a pair of pants, a pair of headphones. So why not a pair of bras?
I love your question. Though one could also argue the question- why not a pair of cups? lol, you know the ones men wear when they play sports,lol.
Or it is called a pair of cups already? I have no idea, I just thought that it sounded funny. If it isn't called a pair of cups already then laugh. Cause really it protects a pair of something and not just one of the equipment is all I'm saying,lol.
I say we start calling bra's, bra bra's! to emphasize the plural function and protection it so plurally provides,lol.
Errm I think the differentiation between singular and plural comes from the origin of the word. Aren't pants derived from pantaloons? Would it not be correct in saying that that term refers in specific to the legs and hence a pair?