First, and to state the obvious, history is to a large degree telling the story of something in the past. Duh, you say! But, how we view something of the past is largely due to our own past and present experiences. Things that have happened and are happening in current events effect what historians chose to write about and how they will interpret past events. And since current events are always changing as time marches on, so do the interpretations and perspectives of historians. For example, since 2001 and the beginning of the war on terror, there has been a marked increase in the number of books written on the subject of domestic terrorism, especially concerning terrorism by groups in the Reconstruction years. Another example, although older, is the Civil Rights Movement and the enormous amount of scholarship that it produced on the African American experience in the 1950s, 60s, and 70s...and continues to today.
Secondly, but along the same lines as the above explanation, is that the people writing history change as well. People, such as Indians and African Americans who before the Civil Rights Movement who didn't have the opportunities afforded to them in some locations, later gained those opportunities and were able to learn the art and science of historical research and writing. Another example of this is women's history. The social changes of the 1960s and 1970s brought many women historians into what had largely been a male dominated field and introduced new perspectives and told new stories that had previously been undiscovered (unfortunately, due to lack a of male interest) or ignored (unfortunately, due to a lack of male interest). The gains that have been made in historical scholarship by including minorities' and women's work has been enormous. Some of my favorite history books would have not been written if the people writing the history had not changed in the 20th century.
Thirdly, as time passes, new sources are found. The great "social history" movement that swept historical scholarship since World War II has sought to investigate history through previously undiscovered channels...by writing history from the "bottom up." History was once written largely only through limited primary sources; letters, journals, diaries, and newspapers, and of course, secondary sources-what others had already written. But historians not so long ago began to "think outside the box," and by using sources such as estate inventories, court documents, and even oral histories, these historians opened up a world of new information. Locating new information of course changed how we saw events of the past, and only naturally new interpretations developed...and in this way one could say history changed. Revisionism, depending on its context can be both a good thing or a bad thing...but that's another post. People discover and donate new primary documents to archives every day, and of course, previously unavailable documents can shed new light on a historical subject that once seemed closed to additional scholarship.
Lastly, and related to the third, is that the availability of research sources have changed...largely through technology...as I mentioned in a previous post. Historians who were once unable to gain access to sources due to limitations in time, or distance, or both, now have ways of getting their hands on them. Secondary source...i.e. historiography research, is now much quicker through technology. Articles can be scanned and emailed; scholarly journals are available online; interlibrary loans are seamlessly processed. All of this makes researching much easier and much less frustrating for the historian, and it allows him or her more time to make critical decisions, and to explore avenues that would not otherwise be considered.
So, there you have it...my reasons why history keeps changing. Hopefully as time goes on we get better in telling the stories of the past and make them more accessible to the public.