Who participates in debates is decided by the people who arrange the debates. They are not set up or controlled by the government. They are set up and controlled by private people, like you.
Sometimes, the people setting up the debate will only invite the candidates they are interested in.
Once invited, the candidates can decide if they want to participate in debates, or not participate. Sometimes the two major party candidates will say they won't participate if third party candidates are present. That prevents the competition from getting their information out to the public.
