Because GTalk integrated with Gmail and with ability to still having access to other friends was much more convenient and they didn't care about who owns their favorite instant messaging network. And majority of their friends were also on Google.

The truth is that only purists will stay, and most people (even tech people) don't give damn about being locked out.

Google also broke things in a subtle way. You could see the person is online, if they messaged you you would get their message, if you messaged them, your message would show as delivered, but never get to them.

So first thing you thought that maybe they are just busy. When you started suspecting something is not right then it made you think that maybe there's an issue with Jabber etc

I don't think the defederation was ever announced, it was more like a bug that was never fixed.