It's a holdover from before reactive styling was really a thing - there was a stage, if you recall, where people were maintaining two versions of their websites, one being for mobile. It was awful, and mobile user agents were bombarded with banners asking to switch to the mobile version, which would invariably break the flow and bounce them back to a landing page. They then had to claw their way back to where they were in the website, if they could. On the upside, it was mobile first, and had more intuitive layouts for mobile devices. I suspect that Wikipedia still maintains this split so as to not break those old web clients, or any other client built on top of the mobile web interface. I bet that if I broke out my old Nokia E63, I'd still be able to boot up that Symbian browser and read a Wikipedia article.
