The further on looks back in history the more information has gotten lost about the events that occurred. It is also harder to know the credibility of available information. This means that some means is needed to test credibility of source material and to fill in gaps in our knowledge. As a result the philosophical assumptions of a given historian play a major role in his testing of credibility and filling in gaps. This is why philosophical assumptions play a major role in studying History.
A perfect example of the influence of philosophical assumptions is an historian who holds to the philosophical assumption of absolute naturalism. Such an historian would automatically discount any historical reference to a supernatural event. Such an historian would automatically dismiss much of the Biblical account regardless of any other supporting evidence.