Comment by stackskipton

Two big reasons I realize,

First reason is MS SQL team read the writing on the wall and realized if they wanted a chance to stay relevant, they needed to support Linux. I'm not sure that play really worked for them but it also gave benefits for number 2.

Second, they had to eat their own dogfood operationally with Azure and hated the taste of dealing with Windows. Linux offered lower RAM/CPU footprint along with much more ease of use with Kubernetes/Containers. Yes, Windows containers exists but as someone who has had to use them, it's rough experience.