Why are most motorcycle trailers built on two wheels? What happens to a load based on a one wheeled design?

Im thinking of building one to pull behind a 150cc scooter to carry camping gear. I think it would track better as a one wheel design, similar to the old vespa and cushman designs. Any help would be greatly appreciated.

Check out N Line's site.
http://www.n-line.com/
I've seen single wheel trailers pulled behind Gold Wings. The hitch is designed like a U joint.
With all the weight being forward of the wheel the trailer would be extremely stable/
I've never seen one unhitched and parked, like a two wheel, though.

The two wheels are for stability of your load,allowing you better
control of your bike, plus the ability to carry more than if you had a one wheel setup.

What keeps it from falling over....training wheels?

when you go around a curve what is the single wheel trailer going to do? you lean your scooter into the curve - what is going to lean the trailer? are you gonna have some kind of semi rigid - hinged connection? i think you need 2 wheels for flexibility and stability.