There are a few things at play afaict:

• the dominant form of payment online, credit cards, are ill-suited to small, one-time payments
  • high UX onboarding costs
  • high transaction fees relative to transaction size
• recurring revenue is better for business
  • most of these auto-recur and people forget to cancel them
  • you can monetize mediocre content that no one would pay for directly and mediocre content is more common
  • you can forecast your earnings
• people tend to do what everyone else is doing and assume there's good reason they aren't doing otherwise, and most people aren't doing pay-per-read