Discourse is not MediaWiki, that’s why. Discourse accepts markdown syntax (and more precisely, Commonmark syntax), not wikitext syntax.
Instead of manually typing a wikilink, you can just link any text with the “link” icon in the edit bar and the URL to the page you want to link. In fact, VisualEditor users already can link this way if they wish, because the editor takes care of converting URLs to wiki links when feasible.
See also: