VR Zone explains Intel's initial Sandy Bridge-E processors don't offer eight cores because this would push the TDP over 130W:
There's another reason, that may top them all though - TDP. Yes, these are really big dies, with many cores and lots of cache - not as many as the Xeon E7 - Westmere-EX - series in the same 32 nm process, but E7's 10 cores and 30 MB cache top out at 2.4 GHz only. Here, we have an expectation that the full 8 core chip should still work at just above 3 GHz right at the announcement - at least in the '3-D workstations only' 150 W version. See the number? Now, well, desktops don't really go with more than 130 W TDP per socket these days, yet the per core clock has to be higher, to justify the migration from older Core i7, and to keep the difference with the LGA1155 4-core SB chips, as well as AMD Bulldozer.
So, to get 3.3 GHz or higher core speed, and fit it all into 130W TDP, a 6 core limit sounds about right for the current C series steppings at least. A future D stepping could enable higher clocks at the same TDP, or maybe enabling all the cores too at a similar speed.