Google Wave was simply too ahead of it's time. It's funny to see how Slack and HipChat are so similar to it. I recall it's performance being poor and people being a little confused by how much it could do. Google Answers was destroyed by Yahoo Answers - you could see the approach they were going for though - a properly answered question can be shown again and again. SO seemed to strike a balance between the two.
GA worked quite well at first. There were some very active users that did excellent work and got paid quite nicely for their efforts. But a vague set of rule changes started to migrate in.
For example, a non-verified observer could comment on a question and pretty much give the entire answer. An official GA Researcher would answer the the question above the line, and the response from Google would be "Well, you didn't add any information to this topic than what was already on the page, so no payment for you". Riiiiight.
There were other little quirks and then eventually Google announced one day it was over. That's just how it rolled with them. I'm chalking it up to shiny things elsewhere that took attention away from GA. I did get a nice fleecy blanket as a Christmas gift one year. Not sure why.