During the initial phase until Bill was the CEO of the company they kept .net framework licensed. Thus making it work only in closed environment of windows. But after the failure or various features (Silverlight) and products like windows phone, and once Satya becomes CEO and his baby azure becomes the backbone of the companies overall earning. They started focusing on services and platforms instead of the product. And hence the phase of open source started with the advent of .Net Core and made .Net framework free.