Enterprise Storage SCSI FC/SAS Performance and Reliability
Note that all modern interfaces are now SERIAL interfaces We use more silicon, but use less cable and connector pins
If we look at the original design criteria for drive interfaces.
Early desktop systems were price sensitive so the design criteria was for a low cost solution. The approach was to develop a command set that used the host processor to control the functions, and integrate only the electronics on the drive.
As things evolved we wanted a higher performance drive, so SCSI developed a solution where all instructions are implemented in hardware, a controller. This has the benefit of higher performance, deterministic latency and does not rely on the host processor.
Requirements continued to evolve so SCSI moved to an enhanced command set that can run on both parallel and serial architectures, but it is still implemented in hardware.