This is more the territory of Cognitive Daily, but I think I want to offer a possibility as to why Science Blogs is bio-heavy, as RPM observed. I think it is because biology is a science which intersects with intuitive cognitive biases we naturally exhibit as humans. I am suggesting that perhaps a content specific element is at work so that our folk psychology and folk biology perks up when biology wanders into their input domains. Now, there is a folk physics, but cutting edge (ergo, bloggable) scientific physics is so advanced and abstractly removed from our intuitive understanding of the world around us that it is often difficult to relate back to something in the “real world.” String theory and cosmology can gain some traction because they sometimes pretend to address deep ontological questions,1 which I suspect is also an innate human tendency (well, at least for many humans). Additionally, astrophysics can be visually captivating. In regards to biology specifically, I have encountered many situations where I am talking about statistical epistasis in a general and abstract fashion and eyes are glazing over, but attention is obtained once I relate it to human disease and medical testing.
In short, biology is easy to make interesting because it stumbles into areas that we naturally find attention arresting. At least for now.
Addendum: Other reasons that I find plausible, a) biology people are more verbally fluent and socially adroit, leading to more bloggers, b) there are more biology people than physics people, so you see more biology bloggers than physics bloggers, and finally, related to b) is c) the possibility that more is happening in biology that warrants comment in public forums.
1 – I don’t think humans are smart enough to ever grapple ultimate ontological issues. Sorry folks, we ain’t as gods even though we bit the apple.