This is because SCSI is harder to implement, but SCSI is more flexible, and ATA was not flexible enough to support first CD drives. So with first "civilian" CD

This is because SCSI is harder to implement, but SCSI is more flexible, and ATA was not flexible enough to support first CD drives. So with first "civilian" CD drives, appear few proprietary interfaces and some used sort of SCSI. Than just made decision, to save resources, make hybrid, using ATA electronics, but partially SCSI logic/software.