I don't lose a tab among hundreds others if I need to reference something. It has all the necessary navigation on-screen (language switch, search inline, table of contents)
Don't get me wrong, wikipedia website is also brilliant: fast and reaponsive. The app is simply adjusted to the platform it's on, including all the mobile interactions you've come to expect. For example, there's no hover on mobile. In the app I can tap-hold a link and see the preview that you see on hover on Wikipedia's site.
I could imagine a nice native app for Hacker News, too