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?"
We know what's their main focus and investments and if you're asking why Bitcoin. Elon understands it and it's his company, he can invest in everything that he wants and making it look like crazy from others' perspective. But those ideas that he's bringing up are becoming profitable to his company. What's a better deal is when they truly have started accepting bitcoin for payments for their products.