To be honest, there really isn't a special reason. It's just our habit to say get IN a taxi. Unfortunately you just have to memorize exceptions like this.

That is a good question. Though you get in many of these things, we say on. I think we are getting on things that hold many people. Train=on - Plain=on - Bike (on for a different reason: because it does not surround us, also like skate board) Taxi=in (fewer people)

taxi 最小！