I believe it's because the water is shallow out to a certain distance from shore, then drops off significantly. The big lahontans are down in the deeper water. Eons ago, pyramid lake used to stretch from it's current location up into the southeast Oregon.
When the lake shrank over time, it became two lakes with the northern lake eventually drying up. The fish that were in the northern lake now live in the streams in the the mountains of southeast Oregon. Because the creeks are so small and dependent on heavy winter snows, the fish are now only in the 6-12" range.
Part of my old job was to help monitor water temperatures the creeks where they lived. Beautiful fish! It was fun to see them migrate upstream into deeper pools of water in the fall