Intel was manufacturing their boards for the 2006 -2008 systems, so they gave Apple preferential treatment in terms of pricing and availability dates (more money in board manufacturing than in the chips ).
In 2009 however, this changed. I suspect the contract dates were different between CPU's and boards, and Apple took advantage of that while saving money per system by using Foxconn (Hon Hai Precision) to manufacture the boards for the 2009/10 systems.
So when the current chips came out (Westmere based Xeons used in the 2010 models), they no longer had early access to CPU supplies. And it's likely this will continue to be the case as there's no indication Apple is going to cut ties with Foxconn and shift back to Intel.