First, can I correct your questions? To ask questions, you need the word 'Why?' (not because)
'Why are 'Get in the car' and 'Get on the bus' different?'
'Why is 'Get on the car' wrong?
The answer :
We use 'on' for public transport, like 'bus', 'train', 'tram'. 'boat'
We use 'in' for cars and taxis.