I'm not sure which edition this thread was referring to, but as for historical reasons, I would have to point to the first edition Monster Manual. Although it was the first release for AD&D, it seems to have been written with something like the five-alignment system found in the Holmes Basic edition in mind. None of the creatures in that book have their alignments given as Neutral good or Neutral evil. The closest it gets is in some entries which give an alignment of Neutral (good) or Neutral (evil), which I read as basically saying Neutral with good or evil tendencies. The Cloud Giant, for example, is Neutral (good 50%, evil 50%) and the text later refers to "evil cloud giants". Notable exceptions are the Larva, whose alignment is given simply as Evil, and the Su-monster, whose alignment is given as Chaotic. All other creatures have one of the five alignments presented in Holmes. The twelve dragons that appeared must have retained their alignments into later editions, even though the nine-alignment system had been introduced.