I think there's a bit of a trade-off here. Shorter lines are indeed easier to read. But they are harder to not read (i.e. if you want to skim over content to find interesting ones). The official HN site makes it really easy to skim down to find the next top-level thread for example.

There's the same trade-off with code line length. Shorter lines are easier to read, but they make understanding the overall structure of the file much more difficult.