You might as well have asked why are decimal numbers ordered the way they are, that is, "four hundred and forty two" is written like this:
442 (least significant digit to most significant digit from right to left).
It might seem counter-intuitive at first because Latin languages are written from left to right, however, if you investigate further you'll find that the modern number system was based on Arabic language (and then developed to be closer to what it is today), And Arabic is a language that's written from right to left. (correct me if I'm wrong though)
So this rule really follows any number system: significance of the digits is in ascending order from right to left.
EDIT: Apparently (as per the comments) the right-to-left convention had even existed way earlier than that: http://www-groups.dcs.st-and.ac.uk/history/HistTopics/Babylonian_numerals.html