Many efforts have been made to construct simpler languages that are easier to learn (e.g. Esperanto), and relatively simple language arise naturally in the form of pidgins and creoles, when people without a common language struggle to cobble one together (a pidgin) that eventually turns into a full natural language.
But, suppose that there was a language much more complex than any other previously known to humanity. One with almost all known phonemes and tones, with both logograms, syllabic and simpler phonetic scripts (like Japanese), with most kinds of tenses and genders every known, with a very large set of words, and so on (i.e. a "kitchen sink" language), i.e. a language that rather than trading off elaborations in one area for another (as most real world languages do) simply are more elaborate in almost all respects.
For example, high literary Latin in the Roman Empire was more elaborate or similarly elaborate in almost all respects than almost all of the immediate successor Romance languages.
In what kind of society/conditions would such a language tend to arise in?
Could such a language arise naturally? And, if not, what might motivate someone to construct such a language (perhaps from one that was already complex)? Would such a language necessarily have are fully elaborated "elite" register and then a "broken" or "common" register used in other settings?
For example, what kind of historical or societal forces caused Cantonese to have a great many classifiers, while Austronesian languages originating with the aboriginal peoples of Taiwan had far fewer?