My guess is that Arc - the lisp language running HN created by Paul Graham - was new, and coding and maintaining a database driver was out of question.
Today, perhaps the way to go would be to use some sort of json webservice interface to a database written in another language rather than writing a driver.
That would be my guess as well. It's one thing to decide you want a simple forum and have it coded within a couple of days. It's entirely another to spend months creating a stable database library and keep it upto date with all the latest changes.
