@SteveRichter: Because most users don't need more than two cores? I think it's just a pure cost/benefit issue. Intel makes most of their money off of the low-end dual-core chips.
Would .NET apps run better on an octo core desktop? All this talk that more apps should be written in C++ kind of hints at the idea that the processors of the future are not going to be able to deliver the power that .NET apps need.