The Himalayan Range is indeed very rich in species diversity. This is primarily due to the fact that it is a confluence of different biogeographical zones. Each of these zones has its unique features, which contribute to the differences in climate conditions. These varying climate conditions, in turn, support a wide variety of plants and animals, leading to high species diversity.

While high rainfall, absence of exotic and invasive species, and less human interference can contribute to species diversity, they are not the primary reasons for the high species diversity in the Himalayan Range.

Therefore, the most appropriate reason is that the Himalayan Range is a confluence of different biogeographical zones.