Implementation details. To the person using the products, they are nearly identical, with one being for web sites and one being for APIs. I see no reason why these should be separate. Please explain.
ASP.NET MVC is approx. 3 years old product and to support the features of the current ASP.NET Web API in an elegant way, lots of breaking changes would occur in ASP.NET MVC. This is just one of the reasons. You might also see this one as another impl. detail but it is the brutal truth.
It's pretty impressive to see people who have no idea about the technology they are talking about. ASP.NET Web API doesn't force REST at all! Go learn what that FX does in the first place before leaving comments like this.
