From my understanding, D6 leagues were created as a backup in case people in some populous BB countries had exceeded the amount of spots in D5 leagues. Unfortunately, this was some seasons ago, and we're nowhere close to having people start in D6.
That and if we had more than 4096 active teams and only five divisions then some active users would have to miss the Tournament. While that also seems like a distant problem now, it's at least much more likely to ever be relevant than the thought of V filling completely.