Now admittedly I don't know how the paid upgrades work behind the scenes, and whether it requires the change to be made manually. However assuming it's automated, then there'd only be an upfront effort implementing the technical changes to enable monthly subscriptions.
If it isn't automated, or is labour intensive, to implement paid upgrades then it also makes business sense to streamline those processes as well.
I can only think of two reasons to not offer more pay options. Firstly the effort to implement the changes (either upfront changes or ongoing ones) are prohibitive.
Or there is a belief that fewer high value transactions is more beneficial then more low value ones. Now this could be the case, however when dealing with a product & service such as wp.com I think that's unlikely. It's likely that users that pay for upgrades are long term users and intend to stay with wp.com, as there are very few competitors to it.
It would enable more impulse purchase as well. For instance the reason I wanted videopress today was because I realised I couldn't upload video I'd taken from my son's sports day. If I could have gone and paid ~$5, I would have continued paying.
The fact that some other online services only offer yearly subscriptions, does not necessarily make it good business practice for everyone.
You make the assumption that changing