Many English teachers describe the writing style of Jack London as vivid in description while also clear and easy to understand. Throughout such books as "The Call of the Wild" and "White Fang," London describes the scenes objectively, with no apparent agenda behind the description; it is this technique that locates his works squarely within the naturalist tradition.
Naturalist writing places a focus on reality and science, setting aside the more pessimistic opinions of realism and instead looking at biological and social causes explaining behavior. "The Call of the Wild" represents the Yukon as it was during the Gold Rush, with unvarnished descriptions of the reality of a cold winter and also of the way that people behaved toward one another during that time period. The fact that much of the book comes from the viewpoint of the dog Buck makes it easier to keep biases out of the writing.
This use of Buck is consistent with the naturalist philosophy, which would view Buck as domesticated and prepared to accept a master's guidance. London never gives into the temptation to humanize Buck, which allowing Buck to understand the motivation of the men would have done. This clarity of description is what makes London's works popular on middle school literature lists.