History rehashings:
Strictly speaking, the ANSI standard for SCSI wasn't ratified until 1986, so being "first" didn't mean much; technically the Mac Plus isn't compliant with the ratified standard and Apple's 25 pin cabling system continued to violate it for as long as Macs were being made with SCSI built-in. SCSI is descended from SASI, "Shugart Associates System Interface", developed by the company in the late 1970's and presented publicly in September 1981; the draft standard was renamed "SCSI" in early 1982, replacing "Shugart Associates" with "Small Computer" in order to render it company-neutral, and while it was indeed used in the minicomputer realm early on it didn't exactly set the world on fire.
The informal standard for hard drive interfaces between the late 1970's and the mid-80s was the raw ST-506/412 "standard" based on the Shugart/Seagate hard drive models of the same name. It's worth noting that most early "SCSI" hard drives were in fact ST-412-style drives cabled to accessory SASI/SCSI daughter boards housing a complete MFM/RLL interface plus microcontroller, making them *considerably* more expensive, complicated, and power-hungry than a bare drive. Since microcomputers place a premium on being physically compact and inexpensive inserting SCSI between the host and the drive would have in most cases translated to pointless cost and overhead, which means that prior to Apple adapting its brain-damaged version of the standard for the Plus the cost-sensitive parts of the industry mostly avoided it like the plague. Most of the theoretical benefits of SCSI with regard to lower CPU overhead are completely wasted when you're running single-tasking operating systems anyway, as most microcomputers did at the time, and in fact Apple's implementation of SCSI fails to leverage most of those advantages on both hardware and software levels.
Arguably by the time IDE came along SCSI *should* have been cheap enough for wide-scale adoption (the host interfaces and drive electronics were getting down to the single-chip levels of integration for both standards), but SCSI was by that time so shoehorned into its "premium" niche it just didn't happen. (Perhaps if the Macintosh Plus and its descendants had been sold at more mass-market price points history would have been different, but it also didn't help that SCSI cabling was confusing and error-prone while IDE was designed specifically to be hard to screw up. Such is the cost of flexibility.)