I don't think our school looks down on upperclassmen rushing, but I think that the reason many of those girls don't get bids is pretty common. Most juniors and seniors have more things that take away from their ability to dedicate the time that being in a sorority requires. Of course, a girl who's a junior and maybe has a job but lives on campus is more likely to get a bid than a girl who *hypothetically* has 2 jobs, a fiance, a mortgage, and a double-major.
**yes, it has happened before