To my mind, it's a combination of factors. A step through design is one, engine on the swing arm is another along with relatively smaller wheels, automatic transmission and the fact that your left hand operates a brake not a clutch.
Not every scooter has all these features, and I'm sure folks can point to one or two motorcycles that may exhibit one or two of them, but you won't easily find a scooter that doesn't exhibit most of them or a motorcycle that does.