Personally, I feel part of the reason numbers are down at Iowa is because they do not have a good marketing/PR system in place to reach incoming Freshmen and transfer students. This year was the first year they were able to actually put information in the University packet that gets sent out to all incoming students, and it was only a bi-fold brochure that wasn't very informative.
Plus, the University is
very tough on Fraternities and Sororities--at least 3-4 chapters have lost recognition from the University or left campus all together in the last 2-3 years. The President is not very Greek friendly (as opposed to Iowa State where the two most current presidents were Fraternity alumni and worked very closely with the Greek Community) and I know that has put a damper on people wanting to join.
There are 16 fraternities on campus with membership ranging from 10 to 70 members. There are 13 sororities on campus. Sorority total is 120 and membership ranges from 30 to 100 members (pre-recruitment).
http://www.uiowa.edu/~greek/