Accessibility, bandwidth savings, maintainability, semantic markup, visual consistency, support for more complex layouts, and removing the need for ugly hacks—remember spacer gifs?
Except... the site works fine like it is. And it's not a mass consumer site. It's mostly for techies/geeks. And it's not PG's core business. YC is.
Believe me, I have at least a few upgrades I wish were made here (fixing that "the link does not exist" bug, and making commenting/editing more friendly like Reddit), but they are not a big deal, we'll survive.
