(Ugh, Flow.) I just noticed that searching for "wg" on Wiktionary finds wikt:MediaWiki:Common.js, but "wg insource:/wg/" does not. Why?
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I'm personally pretty neutral to positive on projects to build threaded talk pages. But I don't have the kind of experience with talk pages that others do. Anyway, that is beside the point.
For your real question: Lame reasons. See T88247.