The trick is in differential mode. The faster serial buses like USB, firewire, ethernet etc all use differential pairs for data. Differential mode offers a lot more noise rejection besides cancelling rise / fall time delays.
If a parallel bus is designed with differential signal, it would be too expensive. But you can get very high data speeds. But as Flatulent said, serial means cheaper cable and cheaper connector. With speeds ever rising, I double we will revert back to parallel buses soon.