beacuse there are only so many Long range high demand routes.
You would put the a380 on the very high demand routes, but again, say you have a route that requires a long range, but dosent have the load factors requireing 400+ capacity (e.g maybe perth-london etc) , then you go for a lower capacity aircraft to service the route.
The number of seats a plane has, and the load factor of a route, plays probally the biggest factor in choosing which plane to serve it.