why is greek life here so exclusive?

One of my best friends is in a mid tier sorority at USCB. She was shocked to hear about how sdsu's greek life is so exclusive and how only frats/sororities associate with those in the same tier as them. Shes in a mid tier sorority, yet they have socials with all of the frats, and the same goes for other sororities. Just kinda wondering, does anyone know when/why sdsu's greek community became so exclusive, and since when rankings mattered so much? It's kinda weird to me to, especially after going there and just seeing how the dynamic is so different and more relaxed.