I've seen this question pop up on every SE site's Meta, from time to time. The answer is always the same:
The core premise of the StackExchange model is that it is not an encyclopedia, meaning there is no editorial board responsible for verifying answers. Instead, the model calls for a site filled with experts (of varying levels of knowledge and fields of expertise) who, through their collective voting, let good answers rise to the top, and bad answers sink down.
An important distinction in that last paragraph is the very fluid definition of good and bad answers. Again, this is neither an encyclopedia nor an academic course. The purpose of answers is to answer questions, and one answer might be more extensively researched and wider in scope, but if people don't feel it answers the original question, or focuses too much on issues that the OP didn't care for, it might get downvoted. An answer with good information but an inappropriate tone might get downvoted by some people, but upvoted by others who don't care about that metric. Some might decide that structuring an answer like an essay, divided into subheadings and paragraphs, might be a good thing and deserve an upvote. Others might see it as unnecessary verbiage.
The second issue, closely tied to the first, is that an answer's value to the site is determined collectively. The premise, again, is that a large enough group of experts will, on average, reward good answers and discourage bad ones. The fact that this is collective, and based on a crowd-sourced system where everyone down- or up-votes based on personal preferences, means that one shouldn't cry foul over every downvote. It means that some people disliked it as an answer, even though they might be fine with the research in it.
Factual incorrectness is, of course, always bad and should be discouraged. However, since the standards for voting aren't as stringent as in publishing, wrong information will sometimes be upvoted. Again, this shouldn't be a problem if there are enough downvotes to balance it out. Given enough users, enough experts, enough votes, a downvote or two won't matter to a good answer.
It's true that History.SE is still a small site, and an answer might languish without getting the attention it deserves, and end up with a negative rank, all votes totaled, when that is entirely undeserved, just because not enough people read it and decided to vote. The solution to that is to popularize the site! Bring in more people, more active readers and voters, and let the masses get the system working.