I think a big part of this is that the psychology of cash and online purchases is very different.
When you're walking down the street with some bills in your pocket and you see a newspaper, this is a low-effort, low-stakes transaction. So you're comfortable making an impulse purchase of a newspaper without thinking about it too much. Thirty seconds later, you've got a paper in hand.
But when you have to put your credit card information on a website, the stakes feel higher. There is some level of risk. Do I trust this company to keep my information secure? Will there be repaying payments? Do I really want the newspaper badly enough to type in the 16 digits? All of this requires just enough cognitive effort that it's no longer an impulse purchase so your natural decision aversion kicks in and you just decide to not bother.
Newspapers spent a ton of time experimenting with microtransaction business models when the web first got big and they were never able to make it succeed. They do subscriptions because it's the best thing they've found so far that actually works with human psychology.