Edit: you asked why. I first saw it at SELF where Chris DiBona showed it to me and a close friend. It was awesome. Real time translation, integration of various types of messaging, tons of cool capabilities, and it was fully open source. What made it out of Google was a stripped down version of what I was shown, the market rejected it, and it was a sad day. Now, I am left with JIRA, Slack, and email. It sucks.
I downloaded the open-source version of the server to see if I could build a product around it, but it came with a serious limitation: The open-source server did not persist any data. That was a complete non-starter for me.
At that point I suspected it wasn't going anywhere. My suspicions were confirmed when I sat near some Wave team members at an event, and overhead one say, with stars in his eyes, "won't it be groovy when everyone's using Wave and..."
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Cool concept, though.