the "great 8" sororities

[IrishPhiSig]

I am just curious...

I read about the "great 8" recently (I really honestly had no idea about Greek life...other than my mom was an ADPi before I joined a sorority. I guess that's good because i think it would have subconciously affected my decision on which house to pledge).

For those wondering, and I'm sure you all know!, the great 8 are:
Kappa Alpha Theta
Kappa Kappa Gamma
Pi Beta Phi
Chi Omega
Alpha Phi
Delta Gamma
Delta Delta Delta
Delta Zeta

What makes these "the Great 8"? My mother's sorority, ADPi seems to have been the oldest one according to our pledge book, but I've been hearing Kappa Alpha Theta was the first. What makes these more special then the others? I am not putting any organizations down, as I know first hand the "great 8" at my school are all wonderful and great girls. I am just wondering why these specific organizations receive this title while others do not.