1 It created the social graph using emails
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.
2 Privacy concerns
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?)
3 The social network was a tab in the email
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.
4 No compelling reasons to join
Just like Google Plus, Google Buzz never gave people a compelling reason to leave Facebook or the other social networks they are connected to.
5 The way it was launched
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.
6 Users were forced to join
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.
7 Users confusion
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.
8 More of the same features
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.
9 The service was only tested with employees
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?)
10 The way people think of email
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.