Rusty isn't exactly in the strongest position to do anything big like that. The Cathy Cluck incident put the frat in a horrible spotlight, Rusty's dad is through with helping out (I know it was financially, but why would he get behind letting a girl into the frat? Plus he seemed pretty disappointed in Rusty overall), and then there's the dormies' revenge highlighting their terrible treatment of girls in the past.
I think people just like Zoey enough to look past the improbability of her joining the DIKs, as well as the idea of her being a female MC and poetry it rhymes and whatever. And if she does join, then what happens? She's the only girl ever, just because a Burgmeister happened to allow it while he's president? It's just too much of a hopeful wish—and I don't even want it either, especially when 9 times out of 10 Zoey won't even get back together with MC.