It would have increased Apple's costs, and had another expense in terms of PCIe lanes.
Currently, there's not an Intel chipset that supports it (there actually is the requirement of a separate controller chip, but the chipset won't handle 6.0Gb/s and would downgrade anyway). Which is why a separate chip using PCIe lanes is required.
Look here; it needs either the ICH10 or ICH10R to accompany it (same goes with the 5520 used in the DP systems). It's the 82801JB (ICH10) or 82801JBR (ICH10R) that actually contains the SATA, USB, and Ethernet controller.