So you can leverage your/your team's existing knowledge of C#/the .NET Framework, which seems like a good enough reason. Lots of Web sites are still built on ASP.NET MVC.
Also, one unfortunate aspect of the .NET community is a lot of people know next to nothing about anything outside the MS ecosystem and have a blindly dismissive attitude toward it (at least until MS latches onto it like they have with Angular and others).
Mono is significantly behind in terms of features supported (essentially the equivalent of two versions), you'll run into interop problems with plenty of libraries, and from what people who have spent a lot of time with it have told me there are a lot of subtle incompatibilities and bugs that eat up a lot of your time. The tooling is also worse.
If you want to do Linux development, might as well just do it instead of using a crippled version of C#.
Anyway, why not Windows? If you're a C# guy you're probably familiar with it, there is no longer the client penalty (given that you can make a SPA or whatever for most LOB apps), and the cost of licenses is likely small compared to labor, especially if you're already a Windows shop.
Also, one unfortunate aspect of the .NET community is a lot of people know next to nothing about anything outside the MS ecosystem and have a blindly dismissive attitude toward it (at least until MS latches onto it like they have with Angular and others).