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. Memory augmentation 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. The reader doesn’t want to jump around or pick and choose—they don’t know enough to do that. They want to follow a clear path laid out before them.
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.