JIT compilations, as well as concurrent and partial garbage collection features, are still a work in progress for Linux and macOS. Microsoft discovered several issues with Windows dependencies for various APIs when trying to port ChakraCore to Linux and macOS, but the company refactored the APIs. At the same time, it maintained backwards compatibility for apps that use the old APIs.
As Microsoft shifts focus to cloud services, it seems not to care as much whether its products use proprietary technology anymore. That means open sourcing some of its software can become a viable strategy for the company, much as it did for Google over the years. An open source approach can also lead to more developers using its tools, regardless of whether they are using them on Windows, Linux, or macOS.
Finally, the strategy can foster goodwill within the developer community, even as the company continues to create various controversies with its Windows 10 decisions - the latest one being its decision to disallow users from disabling the contextual tips and store suggestions (opens in new tab) in Windows 10 Pro.