Back when .NET was still in beta, I was flown up to Redmond to provide some feedback on Managed C++. I specifically asked them why their Socket API was "crippled" in comparison to WinSock (which has some really beautiful mechanisms for handling things like cross-protocol addressing), and the answer I got was "portability to non-Windows systems".
On the other hand the asynchronous parts of the Socket API was modeled after the Windows kernel, which has true support for asynchronous sockets.
That's one reason why the Mono implementation was always suboptimal, because implementing that API on top of poll, epoll, kqueue and the like is not really straightforward. It's also the reason for why the attempts to build alternative web servers are using bindings straight to libevent, bypassing the socket API.
The original plan was indeed for .NET to be portable to non-Windows platforms, but Windows-specific details have leaked in nonetheless.
