Price is certainly one factor, SATA 6Gb/s host interfaces is another. I believe only Marvell offer a third party 6Gb/s interface that Sandforce could use with their SSD. To my knowledge, only Micron/Marvell offer a 6Gbs SSD in the shape of the RealSSD 300. The next one expected is from Indilinx (JetStream).
Currently Marvell SATA 6Gb/s connections are finding there way into motherboards, and i believe AMD have a SATA 6Gb/s solution.
I would expect we will have to wait on Intel with ICH11 before SATA 6Gb/s becomes common place.
OCZ are also exploring SAS 6Gb/s for the Vertex 2 Pro.