To be honest, I don't really know why this hasn't picked up yet. Maybe the scientific community is not very easy to get organized, as researchers are pretty busy with research, teaching, writing grant proposals, serving on committees, acting as reviewers, etc. There have been some attempts in CS with various success, though.
Another factor may be that most research (at least in CS) gets presented at conferences, gets published in conference proceedings first, and then journal publications are mostly an afterthought, and in many cases are skipped entirely. Organizing a conference with open-access proceedings may not be as cheap and easy as setting up a web-site and getting a few well-known names to serve as reviewers...