The problem is that it's very difficult to make badges actually encourage the type of behavior you want, unless that type of behavior is "spend a lot of time doing something meaningless".
For example, HN could have a "1000 Karma" badge, which would be meant to encourage thoughtful, intelligent submissions and posts. People could instead say majority-pleasing things. Even worse, people could collaborate to vote each other's posts up. Get 32 such people together, have each make 32 posts in a topic that left the "New" page without getting any votes, and presto! 1024 Karma for All!
There are many variations on these themes. Badges (in social/multi-user settings) largely fail at getting people to do what you want, unless what you want is for people to waste time getting badges.
Now, in a closed system, this can be minimized: people who would normally read and post on HN are less likely to resort to these shenanagins, and others just wouldn't care. The aggregator gives them a reason to care.