Here's one reason, the conroe chips are the same kind of core. It is actually cheaper for Intel to produce one kind of chip all the time, the more variations you have, the lower your productivity. So for intel it's cheaper to produce the same kind of CPUs, then change the pre-existing chips afterwards instead of fabricating new ones.
They knew these chips were going to be popular, but they were in more demand than they expected. If they decided to make a separate kind of chip for the E6300 & E6400 demand would have been worse (because of lower productivity).
This is similar to what AMD did when they first released their venice cores. A 3000+ venice was actually an underclocked 3800+.
Companies do this so they can have a wide spectrum of CPUs for different prices. Sure they can probably make E6300s like E6700s for only $180 bucks, but then you won't make as much money (and we all know what is on an extremely big time corporations such as AMD and especially Intel).
This also applies to video cards, back in the day when the 7800 GT and 7800 GTX were released, the GT was the same as the GTX except it had 4 of it's pixel pipelines locked and 1 of it's shader pipelines locked (I think). Depending on if you got a lucky card, people could easily flash the BIOS and just unlock the extra pipelines and you got yourself a 7800 GTX. Other cards could do this sort of thing as well.
*Kind of off topic but I feel like explaining this too*
One of the reasons the Russians won during WWII, was because of their extreme industrial simplicity. This is the breakdown: They had only 3 kinds of planes: a bomber, a fighter, and a fighter bomber. That was it; the Germans had hundreds (yes hundreds!) of different variations of aircraft. The Russians also had only 2 kinds of tanks as well. This simplicity is what caused them to out produce the Germans.
(Sorry, I have a test tomorrow in my military history class and this is one of the things I have to study for)