Probably they are trying to take control of Bitcoin.
Yes, i know it´s impossible... until the day they own it all...
I think Gates simply got irritated with the responses to his earlier opinion. He used to be ok with it but earlier this year said that BTC is bad because it enables criminals with new ways to trade drugs and such. People criticized him for this opinion and I'm sure he read those comments and got even more salty that the world not only doesn't react or support him, but laughs. He's becoming a grumpy old man.
If they do not find any value or the use their business can make it they would say such things. But yes btc could help Microsoft or blockchain technology as well like many other business are making use of it. But may be Microsoft would like to create a new thing or something of its own so would like to disrupt the btc first before they come up.