Considering the z68 is a combination of its predecessors, it makes sense that its the better board.
Often people buy P67 because they plan to use a PCI graphics card anyway so do not require the onboard video offered by the z68.
P67 used to be noticeably cheaper than Z68 so for the sake of really only SSD caching + a few other little bits, users just didnt want to pay the extra for something they probably wouldn't use.
Recently z68 boards have almost levelled out in price with only a few ££/$$ between them. Since the Z68 does exactly the same as the P67, including a few extra features like the SSD cache i mentioned earlier, and the Lucid Virtu support - it makes sense to spend $5 or £2-3 more just for a newer, more equipped motherboard.