Both of these have substantial advantages from a merchant's point of view. Subscriptions provide guaranteed income. If a merchant were to choose between selling digital goods on a one shot basis and selling on a subscription basis, the merchant will choose subscriptions. Although the higher price for a subscription will mean that some of the potential buyers will be lost, it also means that a higher price will be extracted from a number of buyers. Moreover, the guaranteed income gives the merchant much greater ability to plan, provision and grow.
Subscriptions have another and perhaps even more important quality.
They provide considerable information about the consumer and establish a relationship between the consumer and the merchant. The relationship also generates renewals, thereby continuing the income stream in future years. Compared to subscriptions, selling individual items on a one-shot basis is a secondary concern. Micropayments are useful as a way to draw in new customers and to serve casual customers who will not pay for subscriptions, but it's unlikely to be preferable to subscriptions. Advertising, too, has been an extremely successful alternative for funding the delivery of small digital goods.