Actually, higher clock rate can be used on serial transmission, thus, transferring in very high clock rates leads to higher transfer rate compared to parallel transmission. SATA provides a transfer rate of 1.5 Gb/s.
Another advantage of serial transmission is fewer wires need to be used. Parallel ports use 40 pin connector and 80 wire cables, while SATA ports use seven pin connector and seven wire cables. So, using thinner cables makes airflow inside the PC case simply.