Few things, in rough order:

• Smaller = less attack surface. You can strip a Linux OS down to only what is needed.

• Open source, so it's can be peered review. There are Unix distros like OpenBSD, that share lot of user space component options, where auditing is a big thing. The whole sunlight and oxygen stops things festering as much. As abosed to things locked in a box in another box down in a cellar.

• Open source transparency forces corporates to be better. We can see what they are and aren't doing.

• Diversity. The is no "Linux", it's a ecosystem of Linux distros all built and configured differently, using different components. Think of Linux as just a type of base board in a sea of Unix Lego bits. There are plenty of big deployments on BSD bases that share a lot with some Linux deployments.

• Unix security is simplier than Windows security, so easer to not mess up.