Its because they are smart, diversification is something the rich don't play with, they knew that anything is possible, any investment can go wrong so even if they have 100 billion in dollars they will take small risk until they have a very big confidence that Bitcoin will do well in future, that's where they go in big.

Just because there are many billionaires in the world doesn't mean they will all be in Bitcoin as investors, many don't see the need and some are just fine with how they make their money, warren is one good example, he is old school and he is very careful around Bitcoin.

Till this day I doubt if he still invest in Bitcoin, even if he did, believe me he goes in with small amount of money, I can't blame him because he might not see what others saw in Bitcoin, still he maintained his richness very well.