Tesla has made about 700 million US dollars through the acquisition of Bitcoin. If they sell cars, they will make more profit than selling cars every year.
Just let it fall into... and still ask "Why Bitcoin?"
The person who find the bitcoin as a investment tool to tesla must be cleverest person. Because in car manufacturing they need to do inverse and get profit after a couple of years, when the sale made. But they can withdrew bitcoin at anytime, when they need of money.