Machines were much slower when this game was first designed, and one of the main aims of the coding was that the game should run fast. There was all sorts of tricks available in those days, and although the Windows front end had been introduced, I suspect that extensive use was still being made of undocumented DOS calls when putting the game engine together. I strongly suspect that refinements like critical hit areas weren't even considered because of the extra computing that this would incur.
The only clue I have is from playing MM9, and I suspect that it's not relevant.
when I was trying to kill the dragon with Poison Cloud spells. If you targeted his torso, the cloud hit him and then passed on to the area behind him and had no further effect. If, however, you targeted his toes, the cloud stayed around the dragon and did repeated damage. So as you can see, switching the target point did have an effect, but it was an artifact of the spell and not the target's response.