They know, therefore, the approach they select will not work for everyone. But there is only so much space and time. There is not enough of either for comparing in detail how basic themes play out over the course of a century. Scarcity and excellence are intrinsically incompatible. Nor is there any reason why students should have no say in choosing the materials they will investigate.
Historians always pick the topics they wish to investigate.
They pick the approaches they wish to follow. They decide which sources to consult. The way to learn to ride a bike is to ride one. The way to learn to be an historian is to do history. We will therefore ignore conventional practice.