It is because there is civil unrest, crime, kidnapping, and terrorism. Also, there can be wrongful detention of US citizens.
Iraq is not safe for US citizens due to terrorism, civil unrest armed conflict, and kidnapping.
Somalia is not safe for US citizens due to crime, murder, and poor health conditions & terrorism.
The poverty-stricken country is totally not a go-to place for US citizens as there you can be kidnapped or asked for money.
Ukraine is surely not a go place for US citizens now as it is at war with Russia.
Afghanistan has been taken over by the Taliban and since then it it not ago to place for US citizens.
Yemen is not safe place for US citizens as it is sickening with terrorism, kidnapping, health risks, and landmines.
In Syria, US citizens may get unfair detention, so it is not right to go to Syria.
Sudan is unsafe due to civil unrest, crime, kidnapping, and armed conflict.
Iran has not so friendly relations with the US and for that reason is totally unsafe for US citizens.