It was more to do with that then being outright dumped. ISU and the AAU have had a tight relationship for a long time, and there was definitely a path to ISU maintaining membership. That path simply is not financially lucrative to ISU vs the current research focuses, methods, and sponsors. The corporate sponsors are also looked down on by the AAU, which ISU has had to employ as a financial resource given the drying up of state funds.
I believe based on some things I was told that AAU membership became less important once the B1G dream was dashed by USC and UCLA reaching out to the B1G.