Why is that only (majority) female graduate students are researching on gender issues? It highlights the role of 'choice of questions for research'.
One's choice of questions is partly rooted in one's own experiences, beliefs and so on. Amartya Sen and Albert Hirschman are good examples. Sen's experience of Bengal's famine made him research on famines and guided much of his work. In recent times, work of Prof. Roland Fryer of Harvard University, on race and education illustrates this. From his experience, Prof. Fryer had an intuition that students of colour feel it uncool to excel academically. That intuition was behind his major work on race and education.
In the aftermath of Trump's victory, one of the after-shock reflections was that, why couldn't sociologists predict this? Responding to that question, a sociology professor pointed out that 'sociology discipline' is losing its diversity in the profile of candidates in its doctoral candidates. In the absence of such diversity, there's a narrow focus only on certain themes and some important themes are getting missed out.
On a broader note, can readily identify four reasons behind the phenomenon of under-researched themes or certain themes going out of radar.
Essentially, you end up having a pool of candidates researching on issues of development who might have never experienced those issues of poverty, gender, hunger, oppression etc. It doesn't mean that one needs to experience these issues to be able to research them. It's just that it's also good to have people who have experienced them because they can bring new insights.