There are multiple possible reasons to explain the Power Brake unit vs. vacuum.
1) Mechanical: The power brake unit derived it’s power from driveshaft rotation. The BW Overdrive interrupts that rotation in freewheeling. The sequence of adapting them would be Engine, Clutch, Transmission, Overdrive and then Power Brake unit. This would be an engineering challenge and costly to resolve
2) Cost: The vacuum setup is a lot cheaper
3) More Owner drivers: The Great Depression wore on causing many chauffeurs to be sidelined from driving in local communities so owners would appear less ostentatious. Also more women began driving. The SW Power Brake unit is extremely powerful except when parking. At 1 mile an hour there is not much driveshaft rotation, the treadle pedal goes flat to the floor with no pressure and the car travels another 18 to 24 inches. A shocking experience or worse for those unpracticed with this characteristic.
I have the privilege of having a 1933 1247 and a 1936 1601 so I can describe both systems:
The SW unit is the best braking system by far of ANY until disc brakes. It has the lag at very low speeds, but that is quickly learned. Fade is virtually non-existent. Creeping when stopped indicates that an adjustment is needed. In 21 years I have never experienced that issue. Locking up all 4 wheels at 70 mph with a 6,000 lb. car is possible
The Bendix vacuum unit is good compared to other non-hydraulic braking systems on the 1st hard stop. Fade increases very rapidly with subsequent stops. Bear in mind that the brakes at the wheels are the same, in my case right down to the linings. In my 26 years with the 1601 I have had a few scary stops due to fade and have learned to drive this car much more defensively.
The best setup would definitely be the SW unit plus the overdrive if they had gone that route