Classical Chinese generally refers to written language based on spoken Chinese, so it looks tall, but it seems out of place in daily life.
The vernacular Chinese is closer to the spoken language, more active and unprocessed, so it looks easy to understand.
Romance of the Three Kingdoms is a novel and belongs to the vernacular. Of course, some plots are also written in classical Chinese, but mainly in vernacular Chinese.
In contrast, The History of the Three Kingdoms is completely in classical Chinese. After all, it is a relatively rigorous historical material, so the language wording needs more processing.
Keep asking if you have any questions.