Why is Rush so Cut-throat on the Sororities' End at Big Greek Schools?

[Peaches-n-Cream]

I am a little confused by the definition of cut throat rush. My school wasn't a big greek school. We had seven sororities including one local that has since closed. About half of the rushees wanted one of three 'top' groups. The ones who were not invited back to them often dropped out of rush. That was 15% to 20% of the women who went through first round. The other four sororities had to COB, and sometimes still didn't make quota. Is this cut throat?