The Economist offers a view on why the timing of Charter 08 limits its chances for success:
Authorities are currently tracking at least two unwanted developments. One is an Internet campaign to bring together parents of thousands of babies made sick or killed by melamine-tainted milk to press for free treatment. Police on January 2nd detained its chief organiser in Beijing, just before he was to take the case to government officials. The second—and more alarming—development for Chinese leaders is a document circulating on the Internet called Charter 08. Released on December 10th on the 60th anniversary of the UN’s Universal Declaration of Human Rights, Charter 08 is a potent political manifesto signed by hundreds of Chinese intellectuals and even some government officials calling for sweeping democratic reforms in China.
The article then ties these actions into broader protests breaking out around the country, such as those staged by unemployed workers and taxi drivers:
[…] As long as protests remain small in scale and do not become wider movements, they do not represent a threat to the central government. However, given that Chinese media reporting on such events is becoming freer and access to information is becoming harder to control, the government feels it cannot afford to be complacent. Newspapers, online commentators and websites are expanding the range of subjects they feel free to comment on, testing the government’s barriers. That is why Chinese censors seem eager to hit the delete button on all kinds of troublesome information. An outbreak of major social violence would undoubtedly lead to a rollback of the trend towards more aggressive journalism.
Likewise, Chinese leaders’ instinct will be to force the people agitating for faster political reforms to fall in line until the economic and social climate improves.