Why are unquoted identifiers upper-cased per SQL-92?

Section 5.6 of the SQL-92 standard contains rules 10...13 per which unquoted identifiers should be upper-cased, so foo becomes FOO but "foo" remains foo. These rules are respected by Oracle , IBM DB2, Snowflake , and ksqlDB but not by Postgres , MySQL or SQLite, for example. The question is, why? In my understanding, the optional quoting of identifiers in a language with a lot of keywords makes sense. Consistent case sensitivity or insensitivity of identifiers would also make sense. But making it dependent on the identifier being quoted doesn't look rational. What am I missing?