Not Enough Mentors
I just went to the MentorNet site, which, btw, my University is not a member of, so I can't use all the resources.
Nevermind about that. One really can't expect any help, can one?
So I looked at the list of "Mentors needed" vs. "Mentors available", and it seems clear that faculty, particularly women, in the 'hard sciences' -e.g. engineering, compsci, etc. are much more committed and available for mentoring than their counterparts in the biological sciences.
We could say this is just statistical: there are many more postdocs in biology than in these other fields combined. So there couldn't possibly be enough mentors to go around.
But perhaps this is part of that 'sadly mistaken' phenomenon- everyone thinks that Biology is somehow Easier- that it's less of a science, that there are more women in it so we're less in need than those in Engineering.
I would argue that because the job market is that much more competitive in biology, we need that much more help, or we're going to have a lot more people on antidepressants, and a lot more PhDs working in fast-food chains.