1 No official explanation
Google did not provide an official explanation for the reasons Google Answers was taken down and instead they just announced its closure. This left the door open for many theories by other people.
2 Competition from Yahoo answers
One of the reasons many people tend to agree on is that Yahoo Answers took off fast, whereas Google Answers remained small in comparison. Losing market share to Yahoo could have been one of the reasons behind its closure. (See Why Yahoo Answers is bad?)
3 People searched for free answers
Google Answers relied on a pay-to-get-an-answer model. Some people suggest that the reason for its failure was that people tried to look for free answers in other places instead of trying to pay for them.
4 A small community
Google failed to build a large community that can keep the site growing at a good rate. This was announced in their blog post about closing the service in an indirect way. This eventually led to undergrowth and so failure.
5 Google wanted to shift focus
When Google announced the closure of Google Answers they said that they wanted to focus on other products. While that wasn't a clear explanation, it was the only reason given by Google during the announcement, in addition to the small community size.
6 Not enough data
According to one theory, Google created Google Answers to get more data to use in search results. When Google failed to get enough data, they decided to shutdown the service. (See Why Google Buzz failed?)
7 A small number of researchers
According to one theory, Google answers could have failed because the service didn't manage to attract a large enough number of researchers, people who answered the questions, and so had to close.
8 It was just an experiment
According to Google's announcement, Google Answers was just an experiment. This experiment could have been done to collect some kind of information that Google wanted.