I am not an expert, but I think the big difference with Physics is that there is a lot of money involved (patents for new drugs, etc). In Physics, CERN and others had the vision of Open Access
http://library.web.cern.ch/library/OpenAccess/ for scientific publications, a mode in which the editing expenses are paid by the authors so that readers can access freely. This was possible because the big publishers were not really given an option: if they said no, physicists would publish on already-existing free places like arxiv.org which give comparable (if not better) visibility. Since there are rarely any immediate money-making applications for Physics papers, an atmosphere of openness could develop and once Physicists got used to it there is no way back.