After the new millennium, the channels between the computer and disk drives have been switching from parallel to serial because of synchronization difficulties. 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. Signals on multiple data lines can arrive at the receiving side at different times and must be aligned in order to turn the bits into meaningful bytes.

Serial Is Less Costly
It is less costly to design ever faster serial lines than to create and build the necessary circuitry to keep faster parallel channels properly aligned. This is why Parallel ATA and Parallel SCSI have migrated to Serial ATA (SATA) and Serial SCSI (SAS). See SATA and SAS.