Why Google Buzz failed?

In the beginning, Google Buzz created the social graph using email addresses. This was not an effective way to create a social graph since people didn't want to follow everybody they emailed. They later fixed the mistake by only suggesting friends based on email activity.

Some people complained about Privacy issues as Google used the information in their Gmail accounts to construct their social Graphs. (See Why Do People Care About Online Privacy?)

The social network was just one of the tabs inside Google's Gmail account. This made it seem less interesting compared to fully fledged social networks like Facebook. 

Just like Google Plus, Google Buzz never gave people a compelling reason to leave Facebook or the other social networks they are connected to.

Google Buzz appeared in people's emails all of sudden. This created an unpleasant experience for many people who either got confused by the service or found it annoying.

Many people didn't like the fact that they were forced to join a service they never opted for. This was one reason Google Buzz was considered annoying to some people.

When Google Buzz was launched, users got confused about what was public and what wasn't. This led to more privacy concerns and gave the product a bad impression.

Google didn't include any new feature that wasn't already present in other social networks. This gave people no reason to stick to Google buzz.

Google tested the service internally with its employees without doing external tests. This prevented them from seeing the reaction of people who don't know much about Google or the service. (See Why Google Answers failed?)

People think of their email account in a certain way. Google Buzz tried to change that image by forcing a strange service to be present inside the email. This resulted in a bad brand image for the product.