Since many wires have to be used (one for each bit to be sent), one wire generates interference in another. This is the main drawback of parallel transmissions like PATA. On the other hand, serial transmission uses just one wire to transmit data, so it is more robustness against noise.
In fact, SATA was developed to serialize the parallel ATA interface which transmits data in a serial mode. It is a very simple interface which supports only point to point connection. Since in SATA at least eight bits per clock cycle will be transmitted, it is eight times faster than serial transmission where only one bit is transmitted per clock.