Im pretty sure the reason they did it was because there is nothing else to put on it. The next step up for Kawasaki is to a FS481V which is way too big/heavy for that mower.
Same thing with other manufacturers in vertical shaft engines. They jump from a 170-200cc product up to a 340cc or larger which is just too big/heavy for that machine.
I think the perfect engine would have been the old Kawasaki FC290V although that might even be too heavy.