Probably because Gates has an huge personal investment arm that hires professional investors who dig into crypto and concluded that it's nothing more than ponzis/FOMO/greater fools/fraud/scams.
Gate's investment company has 100 employees:
https://en.wikipedia.org/wiki/Cascade_Investment
https://www.linkedin.com/search/results/people/?currentCompa...
Also, it doesn't take a genius to come to the same conclusion as Bill Gates.