Cynical me would say it was used a lot at first, but then ASCII became something of the past, and some company doing data work started using commas, it got trendy and ASCII RS went into the closet.
Wikipedia suggest CSV was already around at the time of punched cards, I guess people would prefer commas over some obscure RS code.
Visible delimiters are easier to understand when folks look at the files later -- they are essentially self-documenting. Whitespace takes more work to decode.
Comma-separated values is a data format that pre-dates personal computers by more than a decade: the IBM Fortran (level G) compiler under OS/360 supported them in 1967.
CSV isn't something computer specific, it's basic human grammar, CSV probably just put a tag on a common practice.
Wikipedia suggest CSV was already around at the time of punched cards, I guess people would prefer commas over some obscure RS code.