There are a few things going on between a flat baffle and an H-Frame.
1) The sub drivers put a lot of stress on the baffle. A flat baffle would be moving (vibrations) quite a bit with multiple 12" drivers in it. The sides, top, bottom and dividers of the H-Frame adds a lot of rigidity to the baffle and reduces the baffle forward/backwards/vibration movement. Neither of these 2 designs provide "force cancellation".
2) The flat baffle would have to be much wider and taller to active the same "D" that the H-Frame provides to separate the front and back waves. The flat baffles would have to be @ 26" wider (@ 39" wide) and 13" taller (@ 39" tall) than the H-Frame to approximate the same front/back wave separation.
3) The cubic shaped chase of the H-Frame adds a small degree of load on the drivers which would be less on a flat baffle.
Those are 3 differences to start with.