There are several reasons why the United States did not allow Shah Mohammad Reza Pahlavi to stay in the country. One reason was the Iranian Revolution of 1979, during which the Shah was overthrown by anti-government protesters and forced to flee Iran. Allowing the Shah to stay in the United States could have strained diplomatic relations with Iran and potentially escalated tensions between the two countries. Additionally, there were concerns about the potential backlash from the Iranian public and the possibility of further destabilizing the region. The US government also had to consider the safety and security of the Shah and his family, as well as the potential implications for US foreign policy. Ultimately, the decision to deny the Shah's request to stay in the US was based on a combination of political, diplomatic, and security considerations.