For any historian, even just determining what happened in the past requires a lot of work already. Finding primary sources that clearly describe facts, are reliable, and not too biased is hard, and reconciling contradictory sources is even harder.
Explaining why these things happened, or what would have happened if something else happened differently is much harder still, because you must use this incomplete information to establish causality or (even worse) simulate an actual alternate reality.
Just try for a second to ask yourself what would 2021 look like had 9/11 never happened. It's a daunting task, even though we have a massive amount of information about that time period (having literally lived through it). Doing the same with remote history with historical rigour is essentially out of reach.