It's also a money/safety thing. Drivers are flying higher, and pushing their trucks to the limit of their capabilities each and every show. Debris from the cars, vans, etc can easily be flung places that they were not intended to go, and possibly causing safety issues. It also is becoming increasingly expensive to buy cars, vans, etc. to put onto the course. The dirt is cheaper and holds up better than the cars throughout a show. Unfortunately, it is to some, like me, not as entertaining in certain situations, but as @Swegliner849 said, it's Evolution of the sport, and MJ will continue to evolve.