In his own reddit AskMeEverything page,
https://www.reddit.com/r/IAmA/comments/80ow6w/im_bill_gates_cochair_of_the_bill_melinda_gates/He answers about cryptocurrency stating
The main feature of crypto currencies is their anonymity. I don't think this is a good thing. The Governments ability to find money laundering and tax evasion and terrorist funding is a good thing. Right now crypto currencies are used for buying fentanyl and other drugs so it is a rare technology that has caused deaths in a fairly direct way. I think the speculative wave around ICOs and crypto currencies is super risky for those who go long.
He isn't wrong in his sense, if the cryptocurrency is to have a future, they should be able to create an environment of trust between themselves and the authorities.
We don't know what's in the mind of Bill gates. Always remember that He is one of the tycoon businessman in the world.
He may said it because it is one of His strategy to get more profit, since He knew that He has a capability to convince others, so once the market get down or dump, He can have a chance to buy a huge of it. This is only for my opinion because He is a businessman.