People are mentioning technical & payment processing hurdles, but those are all easy to figure out. Look at the gaming industry, mobile app stores, iTunes, online gambling, toll booths, tipping on various online platforms. Micropayments are most definitely already a thing, and have been for a while. A better version of your question is – why do newspapers charge for a monthly subscription instead of a per-article fee…
In Germany there are sites like "Stifung Warentest" (a mostly neutral product testing organization) that charge one-time fees of 5-10 € for accessing test reports for specific product categories, e.g. mattresses. I bought some of these in the past, they also offer regular subscriptions but since most people are only intermittently interested in specific product tests I don't think they sell a lot of those. So it can…
Their independence/neutrality is the key differentiator.