There's definitely some very interesting stuff you can do with individually priced files, and I might try figuring something like that out in the future, but in this case the point of the promotion is to encourage people to buy the initial story while it's initially on sale, instead of waiting out the clock. I need to eventually turn my work free-to-read in order to continue building my portfolio because I don't have the time or energy to maintain two separate streams of output, but this creates an inherent incentive amongst the less dedicated fans to just wait the six weeks no matter what.
People seem more comfortable talking about games here so as a gaming analogy: It's kind of like how a multi-chapter storygame like Life Is Strange etc relies on the timing and capital of people buying the series when it's still unfinished in order to fund and market-justify completing development, and so they often create some kind of incentive or reward for those who bought in early that pertains to the price or availability of subsequent chapters.