You're making a useless semiotic distinction. The default English-language connotation of the words "the public" is to refer to "lay-people; civilians; people with a non-vocational interest in a subject." As in, Wikipedia is not an academic publication, nor is it an industrial publication, nor is it an esoteric publication. When such interests are incompatible with the interests of people outside of those groups, Wikipedia chooses the interests of the people outside of the niche ("the public") over the interests of the people in the niche. Niches can go make their own websites. Wikipedia is for the average human being — one who isn't thinking "in" the context of a domain, but rather in the context of "common knowledge." One who can't just take a step back and search for "[domain] wiki" and then use that, because they wouldn't know what to plug in for the [domain] part.
See also: the job of a dictionary in defining words, vs. the job of an academic or industrial or esoteric text in defining jargon terms.
> Driven by who? The editors at Wikipedia?
Like I said — demand. As in, analytics data of what users are trying to look up — Google Analytics traffic for "[topic] wikipedia"; things typed into Wikipedia's own search box; etc. The aggregate measure of humanity's expectation of a particular Wikipedia article existing; and the generalization of that into an expectation on whether Wikipedia will cover particular classes of topics.
