Because PowerShell is .NET-based, Microsoft needed .NET on other platforms in order to bring PowerShell to other platforms, said Microsoft Technical Fellow and father of PowerShell Jeffrey Snover.
Once Microsoft got .NET Core to work on Linux and Mac OSX -- through .NET Core 1.0 -- the company refactored PowerShell to work on top of it. The PowerShell Core version also is the one that Microsoft will be shipping with Nano Server for Windows Server 2016, Snover told me during a phone interview late last week.
"Existing PowerShell users who need to manage their heterogeneous estates will want this. People building management tools like us with Operations Management Suite (OMS) will want it. People who want to standardize on a specific set of tools will want this," said Snover.