When people are initially learning about a topic, it’s a particularly valuable time to augment their learning, and in particular their memory: e.g. Spaced repetition memory systems can accelerate the unpleasant early stages of learning a subject.
The Mnemonic medium is particularly well suited to this context because The initial mnemonic medium is implicitly authoritarian in premise: it assumes you want to defer to the author’s authority on the subject and memorize every prompt. That assumption is disproportionately likely to hold for a primer, when the reader may not have many ideas or preconceptions of their own about the subject.
I think it’s also important that the primer be covering a well-established field, so that its table of contents is broadly not up for much dispute or authorial idiosyncrasy. When the field is not well-established, readers will be more likely to second-guess the author’s choice of what’s important.
I think these reasons are in part why Quantum Country worked relatively well.
Q. Why was it important to the success of its mnemonic medium that Quantum Country was a primer?
A. The medium assumes that readers want to substantially defer to the author about what to learn and how; a primer’s the best setting for that.
Q. Why was it important to the success of its mnemonic medium that Quantum Country was about a well-established field?
A. No need to second-guess the table of contents: if readers want to understand the basics of quantum computing, they can trust that this is the stuff they need to learn.