Well the problem with mowing a lawn with a pull behind is one of maneuverability.

With a typical hydraulic drive mower the wheels are always engaged. They can be rolling the same speed at different speeds, standing still. or going in opposite directions. But there is no free rotation state. That means that when you turn a corner you need to do a K type turn involving a reversing turn all designed to minimize the tearing up of turf by a wheel digging in. Either that or you are just going to make ugly little divots every where you turn with the exception of those places where you can get the path into a curve. Then you are golden.

A pull behind would really put a cramp on mowing a domestic lawn with trees and other features.
It's all gotta fit on the mower.