Why is it a bad idea to have only one person in charge of a government

Because than other people would not be able to put there opinions into government like we have here in America. And also the person in charge would only get to what they want and not listen to the people of America.

if only person was in charge of a government, it would gradually convert into a dictatorship, simply because there are no other branches of the government to keep the executive government in check. This is why governments have three branches, to make sure power is balanced. Two branches and even four branches wouldn't work because of uneven balances in power.