Why should historians avoid oversimplification when analyzing sources?

Because historians can leave out important details when oversimplifying things.

They can't miss anything, even the tiniest source.

Explanation:

Historians affront a common problem, which is to prove their hypothesis with credible sources. So, to achieve that goal they can't miss any information. But, they need to segment or reduce their sources, because if they are to many, historians can lose their north, which is pretty easy because the problems related with history is multi-variable.