Because it’s an old volcano, but not so old as to have been eroded. Hawaiian volcanoes go through stages. Kilauea is very active and young. Mauna Loa is also active but not so young. Mauna Kea is much older and went though a late stage resurgence of activity that precedes dormancy. This is the source of the small cinder cones. Kohala is ancient and has been eroded away. Eventually Mauna Loa is go through the same process and make its own small cinder cones.