A Simple Explanation
Parliamentarianism has its origins in the British Isles. As a constitutional monarchy that pioneered representative governance in the early-modern era it remains a formidable defender of liberty, especially in a fight against fascism. The modern United Kingdom is a parliamentary democracy like most other western democracies. Its most distinguishable feature is the lack of an executive head. The leader of the country is the leader of the legislature, which is chosen by the legislature, or parliament. The United States is often referred to as the leader of the free world but has a unique view on how a free country should operate, at least in regard to the other prominent democracies like the U.K. and France. The three of which all encapsulate much of the thinking for liberal democracy.
See here for more on liberal democracy.
Key Concepts
Prime Minister – The head of government, unlike the presidential office, the prime minister is not separate from the legislative body. Different parliamentary systems establish different extremities of independence but, nevertheless the prime minister is the head of state.
Presidential System – The system that rivals a parliamentarian one. The United States is most famously not a parliamentary republic. The president is separate from congress, and they are often at odds with each other.
Main
It is never terrifically clean-cut as to what a political system truly is, and the same is true when it comes to parliamentary republics. This (pg. 6) outlines the differences between prominent democratic systems.
The United States is relatively unique, it is by no means the only democratic country that organizes itself the way it does but when taking into consideration the role that the other democratic powers play in the world past, present, and future the question arises: Why has everyone else chosen parliamentarianism instead of a presidential system? As stated before, governments are not exactly clean-cut in how they operate. Germany for example has the Bundestag that isn’t entirely independent of the executive. But regardless of this and all other seemingly minor details I would guess they, in Europe and in Canada, simply have much of their democratic tradition rooted in the historical evolution of their governance systems. In Europe, parliamentary systems evolved as monarchies gradually ceded power to elected representatives, which allowed for a more seamless integration of the executive and legislative branches.