Billionaires simply mean a person who has large sum of funds. And Billionaires always try to find various ways to multiply and increase their funds. It might be possible that some invest in Bitcoin for better returns and rotation of their currency, on the other hand there are few out there which don't find it important and does not consider it as a proper investment. But, Billionaires are already rich so Bitcoin becomes successful or not it does not matter much to them, Bitcoin matter to those who are trying to increase their saving and have limited resource.
Rich people knows about cryptocurrency that is why when they saw the advantage of having this one, they were curious and study about it and do research, when they have found out its potential return of investment they bought as much as they can, of course they are rich so they are interested in this kind of industry by putting their money with great return at a short period of time.