After the new millennium, the channels between the computer and its peripheral devices have switched from parallel to serial data transfer. Although it would seem that a parallel cable with multiple lines for data would always yield a faster data transfer rate than a single data line, keeping the bits aligned in a parallel channel requires more complex electronics. It is nearly impossible to create 16, 32 or 64 wire traces on a motherboard that are identical in length. Signals on multiple data lines can arrive at the receiving end at different times and must be synchronized in order to turn each set of eight bits into meaningful bytes. In addition, parallel data channels are susceptible to electromagnetic interference between the wires.
Serial Is Less Costly
It is therefore more efficient to design ever faster serial lines than to create and build the necessary circuitry to keep faster parallel channels properly synchronized. This is why Parallel ATA (PATA) was replaced with Serial ATA (SATA), and parallel PCI was replaced with serial PCIe. Parallel SCSI migrated to Serial SCSI (SAS). See PATA