the main reason why he said it negatively due to his ego.
he's also an investor,because of that he needs to maintain his position and makes a proper action.
just imagine it,
if he said Bitcoin is a gold and it will become our future.
what will happen with other assets ? people will sell it for money and buy Bitcoin instead.
if they do that,Bill Gates will suffer a lot because his position will be reduced a lot by those sells.
for now he'll talk everything about Bitcoin negatively,but in a few years time later.
he will support it wholeheartedly.
it is all about time!