Because my mechanic (amongst others) of decades of experience reports how marvelous the dual to single carb conversion is."Transforms then", his words.
Not necessarily true. Due to port biasing, you could end up with a bike that has a much rougher motor. And in my experience, twins carbs have the ability to provide for much more fuel efficient running, ie better fuel economy. It has mostly to do with how the right hand functions, though. Twin carbs make for more top end power and when matched with the proper cams and cam timing, make for a more fun ride.
It's all in the combination. The early 70's TR6's has what I think of as the perfect match, but the Bonnies were good, too. In fact, the move to 750cc's wasn't necessarily a good move or even necessary. The TR6's and Bonnies are more fun to ride than the 750's of any stripe.
My two cents.