Looks similar to what some Lisp dialects call
futures, although the syntactic restriction (plus some of the discussion links) imply that they're doing it via a local CPS (continuation-passing style) transformation.
As to why it's taken so long, if you come from a C/C++ mindset, you probably think "the stack" is real; you expect variables to live there, you might even create pointers to stack-allocated things. Whereas in a higher-level language, you think about the program context (or continuation), which might live on "the stack", but might be copied or moved when the language runtime feels like it to implement things like first-class continuations, lightweight threads, deep recursion, and proper tail recursion on unfriendly platforms. It sounds like .NET is just taking some of the techniques developed for implementing higher-level languages and bringing them, conservatively, into C#.