Because as a dev, my attention is precious. To be productive, I prefer to pull information, than to be pushed information to.
E.G: HN doesn't push information on me. I can answer to you now because I know that at this very moment, I don't need to focus on something hard. But this afternoon, I will, and so I don't want to be disturbed and will just close this tab. Voilà, no distraction.
I don't need to be notified bugs: I don't work on them in real time. In the morning, I select the ones I will work on. New ones have little relevance to me until the next morning, which a dashboard will show me.
I need notifications on things like my clients calling me or my server failing. Those are the stuff that I need in real time.
The very first thing I do when registering with a new service, especially corporate/social ones is to go to the notification settings and disable everything but the direct message stuff.