Because you built a new computer. It would be pointless to buy an OLDER CPU (ivy/sandy bridge).
If you are starting from scratch ALWAYS go as new and current as possible. If you said "hey I have an 1155 z77 motherboard, we would have recommended you get an i5 3570k instead of wasting money on a new motherboard. BUT, because you started from scratch, it would not make any sense to buy something old.