Why not a subreddit?
I really don't think that professional communities really do well as subreddits. I think the way I use Reddit is like many others- once work is done and I want to fool around I'll surf for funny videos and interesting links.
But I think that's a different use than the place HN and DN hold. They're more like a watercooler to talk about the latest things happening to us professionally, or to critique one another's work. That's really important for an industry, be it start-ups or designers or in my opinion dataviz.
I think also that the communities of these sites should be reflective of their userbase, and in more than just having a different subreddit theme. Look at the differences between HN and DN to understand. Those differences are signals to the users that they are 'home' (Helvetica Neue Bold? Seems like my people...), and it requires the site making little modifications that make it better for their target audience as we have been so lucky to see here on DN. That requires something different than a subreddit, I think.
Good point, you convinced me :)