the main point is, if you're after average performance, you have that already, there are dozens of chips from both camps that offer fantastic value and great performance in most things. There ARE dual core phenom's(albeit phenom 1's, that aren't bad at all just have bad press) overclock well and are very fast already.
But getting back to the point, new chips are supposed to be fast, they are for people that want the fastest performance you can get, not generally the best value or mediocre performance, theres dozens of chips around. Releasing dual cores first when most people who UPGRADE want MORE performance. Likewise a new dual core will NEVER, in a million years outperform a quad core in most area's these days. Encoding, multithreaded, rendering, multithreaded, gaming, multithreaded by and large with older games that only use single threads not coming close to maxing out a 3Ghz single core anyway.
Not to mention, the dual core parts are made out of, whats that, FULL quad core parts that have 1/2 cores failed. If yields are good, then the number of failed parts isn't that high and it takes time to build up the numbers of failed parts that can be branded as dual core. Why? because you make a 300mm waifer with say 200 chips of quad cores, of which 30 are available to be dual cores. The problem is midrange sales have and always will be FAR higher than high end parts, if you release a dual core part with only 30 chips available no one will be able to get one, prices will be high and it would be pointless. 6 months down the line and you'll have 20k chips built up for a proper release, they won't be limited parts, they'll be cheap and supply will be good all around.
People who buy value don't jump on brand new things at their most expensive, say a dual core version came out at £120, but in 5 months, it would be £80 and offer better value, to someone not looking for the be all and end all of performance, waiting will make no difference except to their pocket.
Almost every release of every new chip, the first one available is an FX, a black edition, an Extreme edition and they are all expensive, as a newer faster part is released every few months the slowest original part slowly trickles down from expensive, to mainstream to value after a year or two, thats how its always been done.
The OEMs suck up huge quantities of the chips for places like high street stores and branded e-tailers.