Why can a lesson built around one question work better than a packed lesson when the topic is new?

Answer: The new idea has fewer details, so it is easier to hold.

The key terms appear more often, so recall feels easier. — Repeating a key term can make it feel familiar, so this is a plausible guess. Familiarity is not the same as usable understanding, because the learner still has to connect the term to a mechanism. A single-question lesson may repeat less while giving that connection a clearer place to form.

The new idea has fewer details, so it is easier to hold. ✓ — Right. A new idea is handled in working memory, which can keep only a small amount of readily usable information in play. A packed lesson adds many elements at once, so some of them compete for that limited space. A single-question lesson can isolate one relation and let the learner work on it before adding more.

The lesson feels more entertaining, so attention lasts much longer. — Boredom can weaken attention, so this guess has a real point. But a lesson can be entertaining and still overload a beginner if it introduces too many new relations at once. Attention is only the doorway; the new material still has to be held and handled in working memory.