Because it was a sacred site for natives whose ancestors witnessed the eruption and the lake's creation. Also the only population center nearby is next to the Klamath river, one of the largest sources of salmon in North America before the dams decimated them

Fish were only brought by the second administrator of the park. He stole it from the first guy, who he literally fought because he had business interests there and they were from competing political parties. Fish destroyed the delicate ecosystem of the lake. It will never recover because it's impossible to remove all the fish