An i3-2120 (3.3 GHz, 3 MB Cache) already costs $150. The 2500K only costs $225.
If Intel released a 4 GHz i3, it would be priced too close to the 2500K to be worthwhile. Enthusiasts (who else would buy an unlocked K series CPU?) would just buy a 2500K and overclock it to 4+ GHz anyway.