Final answer:
The low number of visitors at Kobuk Valley National Park is most likely due to its difficulty to reach.
Step-by-step explanation:
Based on the information provided in the passage, the most likely reason for the low number of visitors at Kobuk Valley National Park is that the park is very difficult to reach. The passage does not mention the park's size, wildlife, or yearly temperature as factors contributing to the low number of visitors. However, it states that the student trekked all the way to Yellowstone National Park but found it closed, suggesting that the inaccessibility of the park may be the primary reason for its low number of visitors.