why is it so difficult for historians to arrive at aninterpretation of past events that is universally accepted

It is difficult to arrive to a universal interpretation of the past because of the nature of the sources. Mostly written documents, some of them are incomplete, and they contradict each other. For example, in a case of a war there are at least two sides, the losers and the winners. Each of them are going to have a different interpretation of what happened during that war, and historians rely on these documents to do their job. That's why is often said that history is made by the victors: their interpretation of events becomes the official, predominant one. This has a political factor, the winners need to justify their power to maintain it, and depend on historians to come up with a justifying narrative. But the losers don't disappear, they have as well their interpretation and use it as a weapon to show that the power of the dominant class is illegitimate and weaken them.