One size fits all
"Yet when we look at how we buy hardware for business it's not a one size fits all approach. Instead it's common to buy different specifications and capabilities to suit different business needs.
So shouldn't it be the same when considering desktop operating systems?"
Generally major enterprise likes a one size fits all with the desktop as it means they only need one OS skill-set - namely the more common one. Hence if some of your users in the enterprise need windows you may as well give it to everyone as MS will often price it nicely with a site license deal if you say you're considering something else.