Because leaving the scene of a crash is the right thing to do....Why did it take him so long to pull over? He/She was just causing more damage to the truck. Idiot!
The better question is, why is the person filming a traffic accident when he should be paying attention to the road? It's pouring and he's driving down the highway, more interested in filming a collision with his cell phone than making sure he doesn't cause the next accident.
I really don't like rubberneckers.