Why do many recent open access “mega-journals” only cover biological sciences? In recent years, there has been a number of quality peer-reviewed open access journals launched. Some of them are what I would call “mega-journals”, such as PLOS|One and the newly-launched PeerJ. However, most of those mega-journals which boast broad scope focus in fact only on medicine, biology and health sciences (e.g., PeerJ’s Aims and Scope). Why is that so?

Are there other such open access journals with high-quality, focussing on other topics (Physical Sciences, the Mathematical Sciences, the Social Sciences, or the Humanities)? If not, what is the specificity of biology and related sciences?
I think one of the major issues here has been the existence of arXiv in all its various incarnations. Since there was already a major open-source distribution network—even if it eschewed important parts of the publishing process like peer review—it made the need for a PLOS|One-like journal much smaller.

A secondary issue is that many of the journals in the mathematics and physics communities have fairly liberal policies on the use of open-source repositories to distribute papers; for instance, the American Institute of Physics and American Physical Society allow publication within their journals, while still allowing authors the right to "self-publish" using their own version of the materials. Similarly, the American Chemical Society, while having slightly more restrictive policies than AIP or APS, still effectively allow open access after one year of "exclusive" access.

So, ultimately, I think the emergence of PLOS|One and similar efforts is an attempt to move more in the direction that already exists in the various physical sciences communities.  

Edited to add: One additional issue that may explain the state of activities are some of the recent decisions by the NIH to require that manuscripts published with NIH financing (thereby including the majority of biomedical research in the US) had to be made publicly accessible within six months of publication. This is a development that has not been matched by the NSF and other major funding agencies, nor is it a rule in most of the European Union, as far as I can tell (although I believe the UK is haltingly starting to move in that direction).

As a result, there is a strong impetus for open-access in biology and biomedical science that is not matched by the other fields (which might not have nearly as much money and therefore would not have the clout of the NIH).