There would be a real problem charging a per article fee and then allowing that article to be readable 'forever'. Assume a million articles created in a period of time...
Now assume 200,000 customers (*) who then choose to purchase a selection of articles, let's say each has an average purchase per year of 10 items - obviously some could buy thousands of articles and others less than 1 per month.
With just 200,000 buying 10 articles, you have 1 million sales records. With 1 million customers, you could have 5 million sales records. While storage is cheap, these 5 million sales have only paid in 250,000 pounds (at the suggested 5p per article).
Yes, that sounds a lot initially, but that's income over a year, and papers have turnovers (or losses) of millions per year, so 5p would probably not be enough, the 250K pounds takes no account of development costs, or charges for collecting these tiny payments, and the processing to allow for these millions of data items will need web servers and a massive database, which (if the articles are bought 'forever') will increase by millions of records each year, slowing the checking down more and more, and needing some splitting up of customers, perhaps, to keep all their records on a single server (which then needs the database of sales backed up on a minute by minute basis).
Oh, and we have not mentioned staff costs for membership queries, or whether adverts are banned from these articles (given payment is being made for access).
(*) or 400,000 or a million, since access is global, and cost is (in the example) very low.