Because being beholden to a single mega corp is a massive business risk. If MS decides to yank support or they go under then you suddenly need to migrate to a new OS for your entire server farm with all the problems that come with it, like having to port all your custom software to the new OS. Also if the US decides to sanction the country your business resides in than you are also fucked if you run your entire IT infrastructure on Windows. Linux being opensource mitigates all those problems. Like if for some reason you can't use the distro you use you can always migrate to a fork.