Why do PoW algorithms try to find a number of leading zeros instead of something like a minimum threshold? I understand that requiring a hash to have a certain number of leading zeros is what adds the "work" part to PoW, but why are zeros (or any number) used?  Is there any reason that the requirement can't be something like "find a hash with a numerical value that is less than X"?  Is that somehow weaker, or is there another reason?