(The example of the Pomodoro app with the subscription is pretty dubious, though.)
Because this is the era before really any major software was sold by subscription. Even 2008 was a hybrid era where some Web software was already subscription-based. Consumer behavior had already started to shift by 2008.
> for year apps were sold for a flat one-time fee.
This model also makes a lot of assumptions about where user data is stored and who is paying for that storage. Many apps have non-trivial backend requirements to support very normal use cases like "I also would like to access this from a Web browser sometimes." Those requirements cost money on an ongoing basis. One-time purchases are not a good fit for this either. We do not see many one-time purchases of Web software; many mobile apps are essentially parts of a larger whole that includes other modes of access to underlying services.
Why go that far back? The average computer back then cost thousands of dollars, much more than what an iPhone costs now in inflation-adjusted terms. The App Store is not a new invention, it's been around since 2008, and for year apps were sold for a flat one-time fee.
The reason everything is a subscription now is because accountants decided that recurring revenue was how every startup should be valued, and every vendor adjusted their pricing structure accordingly.