Historical perspectives change over time depending on the biases of the writer.
Historians themselves are embedded in time periods and cultures, and the times and cultures in which they live influence their understanding and interpretation of the history they record. Historiography, that is, the writing of history, is a field of study in itself, seeing the work of historians within the different contexts from which they were doing their research and writing. Revisionist approaches happen as historical context changes and attitudes change.
As an example, you might look at histories of the establishment of modern Israel written by Israeli historians in the decades shortly after 1948 as compared to recent decades. The earlier historians painted the picture in strong, patriotic terms, without giving too much attention to the side of their opponents -- Palestinians and neighboring Arab countries. More recent "revisionist" historians have sought to look at the whole picture more fully, including the perspective of the peoples Israel antagonized or displaced as the new nation established itself in the land.
