Why you ask?
1. Traffic regulation. How the fuck are you going to keep people from hovering off the beaten path? How are people going to know there's a path in the first place? Designated hoverboard zones? Hovering traffic cones? It'll take only one douchebag to entertain himself with taking out hovering cones to create havoc. Would passing someone above or below them be illegal? To mark a hover-road would be ridiculously impossible. A strong wind could take a hovering traffic light and hurl it into a window. Any regulating devices would have to hover with the road. All that junk would block out the sun and create really ugly, messy skylines. Not to mention create wonky tan lines.
2. Accidents. Road accidents are terrible. Can you imagine a collision between two hovercars? Not only would you have to clean up the explosion on surrounding buildings, you would have to clean up errant body bits all over the place. And you'd have to clean up any collateral damage underneath the crash. So much mess. No thank you.
3. Parking. Would there be hovering parking lots? No? Then you'd have to pave parking lots anyway. If the parking lots were hovering then you could have lovely gardens underneath where the parking lot would be, but you'd only be able to garden plants of the nightshade family because the sunlight would be spotty at best. And what if your hovercar had an oil leak? That splatter would be HUGE!
4. Hovering school buses. All it takes is a small group of jack-asses to jump at the same time to create a massive accident. No thank you!
Cars and other items should remain attached to the Earth's gravitational pull. Gravity is good. Gravity is our friend.