It seems to be due to stock more than anything else. While there's plenty of stock to hit the retail channel, the system integrated (SI) channel can't be as heavily populated. With the SI segment being huge for Intel it's important to keep all those guys happy and that's coming at the cost of the official Ivy Bridge launch being pushed back.
With the official launch of Ivy Bridge still being a little over a month away we don't want offer the i5 3570k as a normal full review because we could see a little change from a motherboard point of view over the coming month. We doubt it, but it's possible, so for that reason we're keeping this as a preview.
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