One reason I could think of is because it encourages informative titles - I enjoy being able to click directly from home page to article, without having to look at the comments. A lower percentage of posters would explain Ruff in the title if there was a description. (Clearly it's currently not 100% either, that's why I'm here.)

Though this is something that keeps coming up on Reddit as well, I assume there's a reason why HN went with this design.