C# is cross platform, I'd bet money that most .Net services run on Linux these days (Azure runs more Linux VMs than Windows VMs after all) This just fills the client side gap so you can unify the full stack under one language a la node etc

Hundreds of thousands of .NET applications run inside Linux Docker containers.

Orgs that have their LOB software written in .NET and want to migrate to Linux without rewriting it. Avalonia's commercial offering is designed to do exactly that.

My guess would be so that they could make use of a single code base.