Originally Posted by 45nm
I believe it has to do with the Die space available on LGA1155 processors.
Intel Sandy Bridge E (6C) 32nm 6 2.27B 435mm2
Intel Sandy Bridge 4C 32nm 4 995M 216mm2
As you can see adding four extra 'real' cores increased die space by 219mm2. At the same time power consumption jumped by nearly 35W in terms of TDP. I believe the die space on LGA1155 does not allow for six core processors without increasing pin count which is why LGA2011 was introduced. If they could have introduced 6 and 8 core variants on LGA1155 they would have done so. There is also the fact that 32nm processor node was required (AMD managed it on 45nm which is fantastic) before Gulftown could be introduced.
Interesting thoughts, I'd like to add to them.
You can see on a client sandy bridge die shot that the memory controller is the only component above or below the CPU cores. All other components are to the sides. There is also a great deal of dead space beside the memory controller, presumably to facilitate core count scalability (from 4 to 2 cores). If the ring bus cache architecture allows so many linear stops in a row, it looks relatively simple to lay out a six core client type sandy bridge processor. A client Sandy Bridge die is about 208.5 by 103.6 mm, for a die size of about 216 mm^2. A Sandy Bridge Core is about 3.18 mm across, so the die of a hypothetical 32nm six core client sandy bridge would be 272.3*103.5 mm or 282 mm^2.
Even with four extra cores, the die size would only be 348 mm^2. At 22nm this could be less than 220 mm^2. For 32nm, this is only an extra 132 mm^2 of die space, compared to the 219 mm^2 extra allotted to Sandy Bridge-E. The remainder in SB-E is taken up by the extra 25% of L3 cache per core, the 40 lane PCI-E controller, the 4 channel memory controller, and the QPI links.
For a 282 mm^2 six core at 32nm, this is smaller than Lynnfield at 296 mm^2, which fit into an LGA1156 package. I don't believe the die space is what's preventing a six core from fitting into LGA1155. I also don't think that the lack of 6 or 8 core chips on 1155 is proof of their impossibility. I think it's probably a lack of competitive pressure from AMD and a reluctance on Intel's part to have a single DDR3 memory channel feeding more than two cores. The rumors are suggesting that even Haswell will not have more than 4 cores on LGA1150, so I suspect it will be a very long time before we see Intel put more than four cores on a mainstream platform. (I estimate that a 22nm 8 Core Ivy Bridge-E die would be also be smaller than Lynnfield, so again I suspect this is not due to die size, but mainly market positioning.)