Those rating agencies have been given a special position by the government. For example, as this article mentions, the Fed will only accept assets as collateral if they carry high ratings from S&P, Moody's and Fitch. Even an SEC Commissioner admits that the credit rating agencies have acted like Fannie Mae, Freddie Mac, and other companies that dominate the market because of government actions.
In a free market, poor credit rating agencies would go away, because if they lose the trust of investors, there is no longer any reason for companies to use those credit rating agencies. The reason that the big three know they have nothing to worry about is because they know the SEC and the Fed will continue to grant them special status.