At the price they are charging, it might not be economical to charge on a monthly basis because there are associated charges in collecting the funds via credit card either through their merchant account or by some 3rd party.
Supposed that with their cheapest plan at $14/yr, it works out to be about $1.15/mth. If they were to charge monthly on credit cards, after all the associated charges, they might be only getting under 50cents per month. If they charge per year, it might still yield them over $10.
Extra disk space and bandwidth may vary month to month. As your site grows, you would inevitably consume more bandwidth and possibility bandwidth. You can predict such consumption to a certain extend but sometimes you can't.
Say if a PC magazine do a review on your site and provides your URL, you may suddenly see a marked increase in visitors and thus bandwidth for that given month. That may drop to the normal level the next.