It wasn't that owners never bothered implementing it, rather people were actively against it because they felt that no content should be filtered period, and never mind children.
It was actively campaigned against to the point that if you did implement it you were criticized in public. So sites stopped using it and browsers stopped checking it.
Scanning it, it seems PICS specifies an rdf format that ratings agencies can use to define their own rating system, with self reporting (of various PICS implementations?) being an option for sites. I can imagine some site owners would find it a little hard to implement.
It was actively campaigned against to the point that if you did implement it you were criticized in public. So sites stopped using it and browsers stopped checking it.
Some linkes: https://archive.nytimes.com/www.nytimes.com/library/cyber/di...
https://www.nytimes.com/1998/01/19/business/technology-to-le...
https://www.wired.com/1997/02/microsoft-employs-good-clean-p...