Complex sentence: HistoAtlas is not only able to visualize the changes in extent of different countries, but also the events that caused this change altogether, because these things are historically more important than just the change of a border.
Split into simpler sentences: HistoAtlas is not only able to visualize the changes in extent of different countries. But also the events that caused this change altogether, because these things are historically more important than just the change of a border.