That's primarily because commercial academic publishers have been running a racket. Professors and grad students put blood, sweat and tears into their research and reports using your tax payer dollars through federal funding sources. We then turn over all copyright and permissions to the publishers who hold the research hostage behind exorbitant book prices or online access fees. The result, as Ryan at Savage Minds points out, is this horrible cycle where grad students are shooting themselves to publish in these closed access publications to get a job and get tenure to then tell their grad students they need to publish in these same publications.
Thankfully we are starting to see some change. The National Science Foundation (NSF) is now requiring grantees of the Archaeology Program to report and disseminate the results of their research as widely as possible through such data management services as Open Context and The Digital Archaeology Record (tDAR). Many publishers will also grant permission for authors to post their individual journal articles or book chapters (for an edited volume) on their personal websites or Academia.edu--all an author has to do is ask! (copyright addenda also help). Some journals are also beginning to offer web-only open access content.
For my non-archaeologists friends out there though, I wonder: is this enough? Shouldn't the research that you, essentially, are paying for be free and available to you? To encourage this sea change, shouldn't tenure be based on a balance of peer-reviewed publications and mainstream outlets for public outreach like popular magazines or even blogs?