*Checks super secret sources*
<.<
>.>
Okay, Wikipedia.
The term "fraternities", and the female-only equivalent is called a sorority, a word first used in 1874 at Gamma Phi Beta at Syracuse University.
The use of Greek letters started with the first such organization, Phi Beta Kappa, which used Greek letters to hide their secret name.
So it appears its a superiority complex thing.
And put's me in mind of Warhammer and the Adeptus Sororitas.
Who are cool.