I guess it's perceived as a very human-oriented & social subject by many people which would tend to put guys off, being more attracted to logical, 'hard-science' subjects....a massive over-generalisation I know! It's also a kindof vicious cycle, with it being thought of as a female subject, hence putting off guys who would otherwise go for it.
There's no real reason for it that i can see, it's such a broad subject with social bits, biological bits, physics bits, and statistics and practicals- something for everyone. About 15/80 are guys in my year at bristol I think...but then again the vast majority of lecturers & researchers in the department are men, so it clearly isn't the case in all areas of the field!