Why doesn’t anyone eat the sandwiches they’re offered while they wait to board the buses?

None of the waiting Japanese-Americans eat the sandwiches because all are despondent about their future. Additionally, the Japanese-Americans are upset that their loyalty to their adopted country has been called into question. In this charged political atmosphere, no one has the appetite to eat the sandwiches the church ladies are handing out.
In the story, President Franklin Roosevelt has given the army the order to evacuate all Japanese-Americans from their homes in Berkeley. In chapter 4, Ken, Yuki's brother, relates that everyone has exactly ten days to report to the Tanforan Assembly Center.
Yuki's family spends the last day at home saying goodbye to cherished neighbors and preparing mentally for the journey ahead. It is a difficult time for all Japanese-Americans. Hundreds of Japanese-Americans have suddenly become refugees in their own country. They gather at a congregational church and wait for buses to transport them to the Tanforan Assembly Center. A minister tries to comfort the distraught, while church ladies offer sandwiches, milk, and coffee to waiting passengers. However, no one has any appetite, and the sandwich trays remain largely untouched. 
Eventually, the waiting Japanese-Americans are loaded onto buses that are bound for Tanforan.