If it was 2AM perhaps it was too difficult to make out the plate? Also, I'd imagine the witnesses would be more fixated on watching you flying across the street than on the truck driving away.

I once saw a flatbed semi run the car right behind me off the highway. He didn't see it or something. I told the police and they didn't seem too interested to hear about it and told me to move on. I was about the 10th story they took, so I'm sure there were conflicting accounts and they'd started to make up their minds already. Meanwhile the flatbed left the scene. I'm pretty sure that girl's insurance company held her 100% responsible for totalling her car. Easier for the police, easier for the insurance.

bkr