Ooo. The tough questions. Well, I can offer a few ideas, but nothing definitive. Here they are:
1. Between 1880 and 1920 was a golden age for historiography. Most of the technical and conceptual conventions used today were developed then, and actually taken seriously as ways of writing better, more substantial history. They actually used words like 'accuracy' and knew what that meant and didn't mean.
2. Like art, the works of history have schools of thought and 'trends' that come and go, like art's expressionism, pointalism etc. During the 1880s to 1920s, the thread was along the lines of Hegelism. This encouraged works like Toynbee's History and "The Decline of the West" along with the Durants' volumes. The idea was that we can understand history, at times more completely than the contemporaries, and in understanding the history, understand where we are going. In otherwords, history has practical and immediate ramifications. Included in this was the idea that with enough facts, reasonable people will come to the same conclusions.
3. Currently, historiography is not taken that seriously, lost in a miasma of "it's all relative and subjective" and "the facts can mean anything."
When historical fact is relative, then the tools of historiography are so much irrelevant effort. Certainly, the idea that with enough facts, people will agree has been totally discredited. Again, I am talking generalities, referring to a zietgiest and not everyone.
4. Unfortunately for us, back at the turn of the century, historians were writing both to their collegues and for the general population--and being read. Now, academic historians are fairly isolated, and often look down on any histories written for the general population. Read a doctoral thesis and you'll get an idea of the wide gap between the expectations of the academics and the general population.
5. After Hitler's and the Communists' uses/abuses of history in justifying their atrocities, western historians actually became wary to of implying that history meant or suggested anything. Some were actually frightened that it would sound fascist to suggest that history was moving in any direction or actually meant something. Read Page Smith's "History and The Historian." It is an excellent book, but Smith actually says that it is dangerous to suggest that historical conclusions should direct anyone's actions or would actually mean something. He almost apologizes for any such notions and calls for 'existential history', where history exists but actually affects nothing. In such an atmosphere, it is not surprising that the writing of history has suffered. It is getting better, but often because of non-academics, rather than 'trained' historians.