It could be the engaging storylines, relatable characters, or the accurate medical details presented. Maybe a combination of all these factors makes it popular.
The Godly Doctor's Daughter was a time-travel novel. It told the story of a modern girl who time-traveled to ancient times and became the daughter of the Godly Doctor. She experienced all kinds of tests and challenges in her family and finally became a Godly Doctor. This novel had some elements of time travel, but it also contained many descriptions of ancient society's politics, economy, culture, and interpersonal relationships. It was a relatively colorful novel.