> Why doesn't HN open news to a new window?
Because the links don't have '<a ... target="_blank">' in them.
> Was that a design choice?
That's the default for links. You have to explicitly add the target attribute to tell the browser to open a new tab or window. Someone decided on this behavior (the default behavior), so I guess you can call it a "design choice."
> Why would they choose that?
Because browsers let you open links in new tabs/windows if you want (right click, middle click, ctrl-click), but they don't let you prevent the new tab/window if the <a> element has the "target" attribute. In lieu of compelling reasons to the contrary, it's best to let users control their browser.
Maybe the HN programmers also wanted to cut down on the "I have too many tabs open, I need some way to manage them" posts by not contributing to that problem.