Tesla as an entrepreneur and a businessman with a high semce of humor in business and business forecasting (from my own view) he has this idea that the price of bitcoin will go very high in the future, and having not join the system earlier before now, he wants to use all he can to accumulate as much bitcoin as he can before the price goes X. The view of bitcoin in the long run will make him more profits than ever, so he wants to posses as much as possible, so he won't be left behind before that time comes.