Other alternative possibilities:

• Scientists have gotten worse at hiding their fraud, or at judging when to be fraudulent.
• The total number of scientists and publications increases over time, so the total amount of fraud also increases (the original paper is paywalled, but the abstract doesn't say that they correct for that)
• The proportion of papers published each year that enters PubMed rises with time. PubMed was created in 1996. I don't know when the database behind it was created, but probably later than 1950. Then the older articles in the database are the ones that were best remembered. Insignificant articles, and probably ones that were retracted or shown to be wrong, simply aren't in the database.
• Perhaps there is a way for articles to be removed from PubMed if retracted (?)
• Many retractions are done between the article submission but before publication (because the authors or reviewers notice issues), and so before they enter PubMed. Some fraud thus goes unreported in PubMed.
• The Internet's dissemination and archival of information makes it easier to discover fraud and less likely the discovery will be hushed up or forgotten