The heavy topspin comes from the wrist, and wrist movement is restricted on a two-handed shot
Its the same reason you can't hit a slice two handed
The pay-off is better control obviously
How does the top spin from BH come from the wrist?
You actually have much more freedom in the "wrist movement" when playing 2 handed.
That's why f.e. you can hit a sharp cross passing shot on a full stretch from BH with 2 hands and there is no way you can twist your wrist to achieve the same with one hand.
Top spin comes from the whole swing and the grip on the racquet.
The fact that usually one handers play more top spin from BH than 2 handers is because of the much wider amplitude of the swing with one hand, which is limited when you hold the racquet with two hands.