Here's why. Timothy Messner-Kruse, an historian at Bowling Green, is the author of two books and several articles on the Haymarket Riot. But when he tried to correct some errors in the Wikipedia article on his subject, he found his changes reversed, not because they were wrong but because they didn't reflect the consensus of opinion. And the opinion solicited here is represented by secondary sources, since Wikipedia (albeit for understandable reasons) discourages the use of primary sources.
The problem is, or course, that facts don't depend on a consensus of opinion. They depend on sources, which in the case of historical writing are original documents that can be examined and cited -- things Messner-Kruse had done, in some cases making him one of the first researchers to do so. But until his findings are more widely disseminated, they won't count so far as Wikipedia is concerned.
Honestly, we can't get too upset with Wikipedia. If they didn't have some policy like this one, they would be flooded with obsessive-compulsive cranks, swamping their servers with "evidence" for alien abductions and Fortean phenomena. But that doesn't mean we actually trust their second-hand facts, either, and neither should you.