The WGA series (7/9) have a familiar venturi. They tune easy and with their moulded venturi have positive fuel draw. The next size step up in carb series is the WB. It has a machined bore with almost no venturi. I would conclude there really isn't much in the way of a variety of Walbro's to choose from in the 150cc size and up and none are going to be designed for airplane use. So they have to make do with whats available.
Engine builders also have to take into account, needle placement, throttle, choke and fuel supply locations, further limiting choices.