Because in Xamarin being that this license does think twice.
"If you are an enterprise, your employees and contractors may not use the software to develop or test your applications, except for open source and education purposes as permitted above. An “enterprise” is any organization and its affiliates who collectively have either (i) more than 250 computers or users or (ii) more than one million US dollars (or the equivalent in other currencies) in annual revenues, and “affiliates” means those entities that control (via majority ownership), are controlled by, or are under common control with an organization."
Xamarin is indeed not supported anymore. With the release of the first CRYENGINE 5.5 preview we're now supporting Visual Studio 2017. To use Visual Studio 2017 you do need to install the CRYENGINE extension for it though. You can find instructions on how to install this on the C# Programming page.