I'm pretty sure SATA SSD performance is bottlenecked by the SATA bandwidth. It's the same flash inside.
Perhaps, but the PCI card also contains it's own controller instead of using the existing SATA controller in the box. Not a complete justification for the added cost, but even if the flash is the same, getting it connected is more hardware than a SATA device needs.
Ultimately it doesn't matter why the performance is higher. Also, the Intel 750 has 18 channels while most SATA SSDs have 8-10 channels.