Nvidia just cancelled its mainstream Fermi due to the fact that GF104 based GPU’s will be already out really soon and mainstream chips will be based on the GF104 architecture.
There were two other chips apart from GF104, GF 106 and GF108 on which the Mainstream Fermi was to be based and they were Direct X 11 Compliant and had a release date for August. But the chips which were to have half the shaders of the current Fermi have been cancelled due to high TDP. Other than that, GF104 has been released so soon that mainstream cards will now be based on the newer and better architecture for better performance.
Although the GF104 is 6 months late from ATI in the mainstream market but still people want to see how nvidia tackles ATI with its new architecture plus notebook manufactures also seem interested in the GF104 chipset.