Small four wheelers are super sketchy at scooter speeds. The short wheelbase makes for quick and flighty steering and the narrow track makes them tippy. You have to lean to maintain balance, but leaning is not initiated and controlled via steering like it is on a two-wheeler, so it feels less natural.

You also have a big increase in cost and complexity. Twice as many wheels which means more bearings, more suspension components, etc. You need a differential and drive shafts and CV joints, which are expensive. A solid axle would be cheaper, but a solid axle on the street makes for some tricky riding dynamics. It is heavier, so the parts need to be stronger. The motor is harder to get at, which makes maintenance harder.

And no matter what, a scooter will always be smaller and so will always have better places to park.