All comments can be summed up in this:
Consistency of staff. The longer a team (and head director) work in the same location, the more they can perfect their method and improve upon the success of previous years. In other words, there are no setbacks from year to year. Look at the bands you named and this proves to be the case. In the case of Bell and Duncanville, they have had relatively recent head director changes, however, those directors had worked alongside the previous head and the transition was smooth.
My 2 cents.