ASP.NET MVC is developed with an annual cadence, unlike the .NET Framework, which releases major new versions every few years. Associating ASP.NET MVC with the .NET Framework would put severe restrictions on the schedule and functionality of MVC releases without offering tangible benefits to customers.
Think about it in much the same way that Silverlight was released with a few year cadence. Part of the flexibility of Silverlight and the speed of adding new features is due to the fact that it is an out-of-band release and is not tied to the .NET Framework itself.