A system of government in which the power to make and execute laws is held by a parliament. Britain has a parliamentary system of government, one of the oldest in the world. The United States does not; its legislature, the Congress, passes the laws, and a separate part of government, the executive branch, carries them out.
And if you look at Britain in comparison: it had a parliamentary system at that time.
From the start, we were not a parliamentary system, in which loyalty to the party is paramount and demanded.
Thailand is a unique society in the modern world: a Theravada Buddhist monarchy with a parliamentary system.