I am not 100% sure of the exact reasons that Ford had for the '36 wheel design. I do know that the '36 wheel design is five pounds per wheel lighter than the '35 wires, for a weight saving of twenty-five pounds per vehicle. Twenty-five pounds per vehicle is huge when you multiply that by the number of vehicles Ford made, $$$ saved.
I read an article years ago that the wheel design, including the bolt pattern improved the handling and ride of the '36 Ford..
Can't remember what the origin of the article was.
RalphM; The '35 wire wheels will fit onto '40 and later drums, and they will fit your spindles if the spindles were converted to '40 and later, which most people use for hydraulic conversions, however, the '40 and later drums do not properly support the wire wheels. I believe Speedway sells an adapter kit that will support the wire wheel hub.