I believe it's something to do with X58 chipsets having an extra PCI-e 16x lane, so there won't be any bandwidth issues. Apparently SATA III/USB 3.0 use some PCI-e lanes on P55 motherboards, and you need a specific controller or more lanes to overcome the P55 board from having to lower the bandwidth or disable its PCI-e lanes. TBH, I'm not fully sure, but something to do with that. I could be wrong though...
I wasn't sure about that either but after looking at the Core i Series Chipsets specs on http://en.wikipedia.org/wiki/Intel_chipset#Core_i_Series_Chipsets
it seems that you are right.
X58 has QPI with a bus speed of 6.4 GB/s and 36 PCI-E 2.0 lanes vs P55 which has the old DMI with 2GB/s and only 8 PCI-E 2.0 lanes.