Why does playing devil's advocate loose you HN karma?

As for why you get downvoted, unless you're very good at communicating a post playing Devil's advocate looks pretty much exactly like a post espousing a contrary opinion for real. In other words, people can't tell you're playing Devil's advocate and they take you seriously.

I fear for a society that no longer collaborates and grows but rather shouts into its own comfortable echo chamber.

Ideally, yes.

But HN readers/commenters are human, just like those on other sites. It's a slightly more curated Reddit, with a less meme-y marquee, but the downvote button is abused just the same.

It doesn't stop me from playing Devil's Advocate sometimes, though. I welcome the intelligent responses when I get them (and there are plenty of intelligent people here to provide them), and usually when I get downvoted without comment I take comfort in the fact that somebody has had their worldview challenged and is silently having a temper-tantrum somewhere.

I certainly don't intend on it to be trolling, however. I think an intelligent, civil debate is its own reward...even if one is outmatched. :D