Except SATA (and especially SATA2) transfer data FASTER than PCI. Which is most likely why they do it that way.

1. is SATA faster 2. the compatibility is unmatched with such a solution its awesome that PCI only have no chance or do you know anyone with a RAM disk card ? but whith this solution anybody can own one and who uses all of his PCI slots ??? if you do so you may think about rebuilding the PC and about the PCI transfer rates .... you can easily beat it with 3 RAID 0 disks ... so it would be a wast of money to use the PCI bus .... while such a card for PCIe would make very much of sense