They're just conservative about changing things, and at this point, the look of the site has more or less become part of its brand.
It works, you can upvote / downvote. It's simple, doesn't need tonnes of JS to run. It's fast.
What do you think needs updating?
The only possible thing I would think about changing is favouriting a post. It's not obvious to me that you click the the x minutes ago part to get to a screen where you can favourite.