coz the factory has lots of factors to be considered:
1.the heat it produced(higher frequency produce more heat),since stock coolers are generally not good enuf especially for quads.
2.the amount of power it draws,the higher ur frequency, the more current it draws from TNB(very true during OC)
3.the warranty it gives u,manufacturers need to assure close to 100% stability(no system in the world has 100% stability) and good life span of the chip.
4.Quota of production.Example:intel feels tht not many Kentsfield procs can pass 3.0Ghz(they test it throughly before they brand it, thts y u get Q6600,Q6700-->its the same chip) while maintaining the same TDP of 130W with all other Kentsfield proc,maintain the same Vcore used by Q6600 and maintain the same amount of stability and predicted lifespan of Q6600. its true, some might be able to do it but the quantity wise is very little,how to sell the product then considering additional cost of packaging for new variant of the proc and other factors. like dat might as well scrap the plan for 3ghz version.

i've read news about intel bringing back the hyper threading technology in P4 days to its upcoming CPU production.quad core become virtual 8 cores.just wish more applications will make use of so many cores.