The point is: When SASI was en vogue (and that was only a very short timeframe, because technology was soon superseded with SCSI and the simpler ST506 and later ATA standards), hard disks weren't very popular for low and mid-range computers.
If you look at early hard disks, for a short time (a time frame of maybe 2-3 years in the early 1980s) SASI was the only relevant standard, so market share must have been close to 100%.
Development of early hard disk interfaces for small computers was simply so short that technology developed much faster than standards could really establish themselves in the market.
Some relatively popular computers like the Sharp XC60000 and the Commodore early hard drives for the PET (D9090 and similar) used SASI hard disks.
Note that the change from SASI to SCSI was only a name change, there were no technical changes applied to the interface - So starting in 1981, SCSI and SASI were effectively the same thing, just with a standardized name. Seen from this end, SASI was actually very popular, just under a standardized name that removed "Shugart Associates" from the abbreviation.
Wikipedia knows the following:
Until at least February 1982, ANSI developed the specification as "SASI" and "Shugart Associates System Interface"
However, the committee documenting the standard would not allow it to be named after a company. Almost a full day was devoted to agreeing to name the standard "Small Computer System Interface", which Boucher intended to be pronounced "sexy", but ENDL's Dal Allan pronounced the new acronym as "scuzzy" and that stuck.