There are a few reasons I can think of that I'll go into:

Technological -- the tools that anthropologists, historians, and archaeologists have today give them a gigantic advantage over scholars from 100 years ago, let alone 1000. Scholarly databases, carbon dating, all sorts of imaging technologies, social networking programs, and modern computers in general means that much of the grunt work of "doing" history can be handled with technology. It also means that infinitely more information is readily available to a historian now than there was even 30 years ago.

Time -- closely related to technology, the fact of the matter is that each of those disciplines I listed build off of previous studies; new approaches, new discoveries, and simple refining of previous arguments means that, in general, knowledge of a subject will improve over time. A work of history is only as good as the sources allow it to be, and new insights into those sources (to say nothing of new discoveries) can produce better history.

Purpose -- this one gets a little trickier, but without going into a ton of detail, history in the western tradition shifted towards the "scientific" (that is, based heavily on factual accuracy, backed up preferably by documentary evidence) in the 18th and 19th centuries. Prior to that, factual accuracy was oftentimes secondary in value to the moral or philosophical truths that the historian wished to convey. Since the modern historian who wants to produce a "scholarly" work must, at a minimum, to show that the argument that they are making is based in reality (with cited evidence to back it up), said historian tends to put far more of a premium on providing as much proof as possible for their assertions... and since, as a culture, we tend to associate historical truth with factual accuracy, contemporary history feels much "higher quality" to us than, say, that of Bede.