The simple answer- It has 4 wheels, just like a car.
I looked into this a few years ago when I bought my quad. There were a couple of years back in the early 2000's where you could. Basically because they have 4 wheels they are held to the same safety standards as a car. So that would mean that they would have to meet the same crash tests and all that kind of stuff. There is obviously no way they could do that, hence why you could never get one "safetied" for the road. I was talking about it to a co-worker back then, and he told me there was a guy he knew in Burlington that got his street legal and it was grandfathered in. I know my co-worker rode with some guys here, so perhaps they may know the quad guy as well.
A "dirt bike" (or dual sport) is held to the same as a motorcycle, and that is why with the right lights etc. you can get them done for the road. But even that changed in the past couple of years. Now if the bike is green plated, I believe it is either impossible, or next to impossible to get it blue plated (street legal).