Um, because it's stupid to use sata 300 in a notebook.
The drive does not perform anywhere near the limit of the sata 150 interface even, so a faster interface will gain you nothing at all. Nothing except shorter battery run time. That's right - sata 300 uses a LOT more power than sata 150, just to operate the interface. Kind of stupid to consume more power and get nothing in return, no?
And how many Watt`s extra does the difference make then? Got any real life comparison to this fact?