When rabbis write books, their intention is not always to make their material widely understandable. Different books have different purposes. Books aimed at beginners should certainly be written in a way that's easy to understand. But many books are written with an assumed level of prior knowledge. This allows writing to be more concise since every concept need not be explained.
This can be extended to more extreme cases. For example, writings on more esoteric topics may use jargon that is completely unfamiliar to most. If every word of jargon were to be explained, the flow of the book would be interrupted, actually making it harder to understand for the book's actual intended audience: those with deep knowledge in the area. These books aren't intended to be understood by a wide audience.