During the development of both SCSI and ATA devices, the design criteria was different for both. ATA was primarily designed with cost in mind, for low duty cycle environments such as desktop PCs and laptops. SCSI was developed for high end systems requiring higher performance and higher duty cycles such as 24/7 operations.
From this we can see that ATA drives used the host CPU to help reduce cost, whereas the SCSI devices used a hardware controller which in turn minimizes the host CPU load.
The reliability of SATA drives today means that they are also suited to higher duty cycle environments, although they still offer lower rotation speeds which means they are not typically associated with Enterprise class applications which require faster, random I/O operations.