The correct answer is for the same reason that a pair of pants or pair of trousers is called a "pair". Both panties and pants derive from the word pantaloons. Originally pantaloons were a covering for the leg only - two legs, hence a pair.
... which leads me on to something rather special. Prior to releasing Blood Music, Chapterhouse asked ambient house/dance production duo, Global Communications, to remix the album. Very little of the original survived ...