The Greek system has had to weather changes to its operating environment. Although undergraduate headcount has been relatively stable the demographics within the headcount have changed over the past 50 or so years. There has been a flip of the male to female ratio until it is now decidedly in favor of females. And increases in minority and international enrollments have placed further complications on efforts to maintain fraternity membership levels. Although fraternities have have had success tapping into non-traditional pledges the reality has still been a declining number of potential members.
The recruitment pool for sororities has tended to break even over this same time, what with greater absolute numbers they have less traditional candidates.