For many years, since the 1980s and 1990s, the Chinese government has totally cut off all communications related to the Uighurs' problems and what is happening in Xinjiang.
After September 11, 2001, when the international community committed to fighting international Islamist terrorism, China very skilfully portrayed what it was doing in Xinjiang as part of international efforts to combat terrorism. In a sense, it used the international community's actions to cover up what it was doing in Xinjiang.
The Uighurs were really having a hard time getting out of China. To leave China, you need a passport and authorization. The Chinese confiscated the Uighur activists' passports. This was before the Internet made it easier to communicate, so people knew very little about the situation. The international community was so focused on fighting Islamist terrorism that it somewhat missed what happened. People were also much more focused on human rights issues in Tibet, so the situation in Xinjiang got a lot less media coverage. In addition, the problem was not as big as it is today.