Because top brushes are a PITA. I have 3. One tunnel with 2 and one with one. If I have the space I'd go with 3 mitters. 1 FTB and 2 STS or favorite setup has 3 AVW with 2 in one direction and one in another.
Top Brushes have issues with Pickups, Taxis with roof top ads - Big Time, and all...