Powershell addresses a deficiency in Windows, in terms of it's command interpreter (CMD) not being quite up-to-snuff when compared to things like bash. While Open Sourcing and making the product cross-platform is neat, it's not going to provide a lot of value to existing *nix users (and admins), as they already have a much better shell (in terms of being a good shell for *nix systems) in the form of bash, since it adheres to the *nix philosophy.
A lot of the Open Source "moves" Microsoft has been taking- The .NET Roslyn Compiler, then .NET itself and now Powershell, and even the ability to integrate bash into Windows- are likely motivated not by any end-user consumer desire but by Microsoft's own desires when it comes to Azure.