Well, not only driver development but also controller development. SATA is a standard where you can go buy the standard components. The connector probably has the same data lanes as a SATA connector, just a different form factor. Most of the space saving come from not having an enclosure like harddrives do - something which only words with SSDs since they have no moving parts and are thus dust-resistant.
Most SSD <-> SATA interfaces are themselves essentially a bridge. You could instead directly wire in to the SSD's interface. It helps in a number of ways, as the SATA protocol was really not designed with SSD's in mind. Under the covers, SSD's are kind of this weird hybrid of RAM and storage devices.
