Because in 5 years of development now, C# has only shipped two major features: dynamic and async. Both of which F# has had since 2007 (maybe dynamic was 2008). Both of which F# implemented better (as libraries, so you don't need intrinsic compiler support). Async's design comes from F#'s design.

But without comparing it to F#, Microsoft has not polished the C# 3.0 design. Expression trees are still half implemented (no way to call one exptree from another). MS created anonymous types, but neglected to add simple types like tuples (let alone anything more advanced). Type inference is still ridiculously limited in scope:

  - Neither parameters nor members can be type inferred.
  - Local function vars can't be type inferred (because they chose identical syntax for expression trees).
  - Generic constraints can't be type inferred.
  - Generic parameters have limited inference. If one parameter can't be inferred, you _must_ specify all of them.

On one of the developer blogs, they noted that the reason fields couldn't be inferred was really due to internal compiler codebase complexity. Yet 5 years later, that's still not taken care of. Basically, they aimed for the LINQ outcome, and implemented the required feature set only within the context of LINQ, not for the language as a whole. Which is fine for V3. But then they just left it at that.