Wikipedia's mobile site was originally running completely separate software (https://www.mediawiki.org/wiki/Wikimedia_Mobile_engineering/Ruby_gateway https://ww

Wikipedia's mobile site was originally running completely separate software (https://www.mediawiki.org/wiki/Wikimedia_Mobile_engineering/Ruby_gateway https://www.mediawiki.org/wiki/Wikimedia_Mobile_engineering/...). My guess is that it was easier to set up that way at the time, and then the configuration was kept when a mobile version was integrated into MediaWiki itself. There are plans to merge the domains (https://phabricator.wikimedia.org/T214998 https://phabricator.wikimedia.org/T214998), although they are vague and stalled.