The question is incomplete and the full version can be found online.
Answer: Because he was unable to randomly assign participants to join a fraternity/sorority.
Step-by-step explanation:
It is possible to study private organizations, like fraternities or sororities, as long as the required permissions have been obtained.
It is possible to measure body concern: the Body Concern Scale.
The issue is not that more people or an equal number of males and females should be recruited. The actual impossibility is that the researcher can not randomly assign participants to join a fraternity or sorority, because that is a major and very personal decision. So the researcher had to work with those who wanted to join and actually got in.