Here's my immediate, knee-jerk hypothesis:
Sexism has appeared to morph over the decades, and while it still exists in its most overt, blatant forms in many ways, there is now at least
some taboo against it in the dominant discourse ... kinda. With this shift, sexist attitudes and beliefs have had to become more sophisticated and subtle, but boy oh boy do they march on with vigor. One of the most insidious ways that sexism perpetuates is in the form of benevolent sexism, where women are seen as magical, nurturing, social creatures (who are also fundamentally weaker and must be protected). These attitudes strike me as being more prevalent now than the hamfisted, hostile sexism, but they are certainly still destructive. (If you ask me to provide support for this, fine, whatever, I'll cite my ****. But I wont do it until I'm called out for it. Because I'm lazy.)
I've encountered this problematic attitude over and over again. In fact, I'd be willing to bet that it exists to varying degrees in the vast majority. I'm not sure why it surprised me to come across quite a few people in this field (men and women) who hold onto a belief (or at least a suspicion) that women are inherently more nurturing and emotional than men. This belief in such a fundamental, biological difference is unfounded until we can eliminate the overwhelming societal pressures for either gender to be a specific way.
With that yada yada'ing about systemic reinforcement of traditional gender roles, I think that psychology is one of the few scientific professions that women can enter into that does not appear to completely challenge society's predominant view of them. Because it's expected of women to nurture and be social creatures, it's become a relatively easier path for women to pursue in the scientific world (thanks to the struggle of many strong women before us). That being said, as wigflip pointed to, the actual power structures within the field are less convincing of a shift in traditional power dynamics. The gaps in these disparities appear to be shrinking, thankfully, but at a much slower rate than I would like!
Perhaps once we smash the patriarchy, males will be encouraged by society to embrace their nurturing/empathetic capacities, and thus look to pursue more careers in psychology. So! If you'd like to have more male companionship in the field, might I suggest you take this hammer and smash with me?