It's the price of the license, not the hardware that's stopping the numbers being higher than they are. I know more than a few people who use MOS on occasion, but are unwilling to shell out $200-ish for a license (that's a full "modern" computer price).
And before the usual suspects get their panties in a twist, this isnt a criticism (I like MOS, and its much more attractive than OS4.x to me in regards to pricing), just a simple truth (from someone who has no interest in playing favorites). In reality there's more people using MOS than the registrations suggest, its just that not all are willing, or able to afford a license.