A Stack Overflow page I'd imagine would be so much more complex than a Hacker News page to render. Would likely require lots of complex queries for everything going on around the primary question and answer mechanism. By contrast Hacker News seems fairly simple.

The file system.

These are the results of different constraints. SO wants a slick, interactive interface with a lot of access to long-tail content. HN wants inexpensive and easy-to-maintain that still allows discussion of active topics that fade from view fairly quickly.