that's a good question, and as I'm sure you probably know, the difference between "on" and "in" can be tricky sometimes.
All I can say is that in this context, if you were to say "he escaped on the car", it seems like you're saying "he escaped while being on top of the car", which logically doesn't make sense.
"he escaped in the car" basically means he escaped using the car, but depending on the content it could also mean that he was inside of the car when he escaped.
If that explanation was bad, I'm sorry. I would be more than happy to try and explain further.