Guessing it largely has to do with it's status. I've only been to airports in Chicago and have never heard of this building. Only know of the Hancock Building and Sears Tower. You can't start renaming iconic buildings and expect people from elsewhere to know what they are called.
It's not as architecturally interesting as some of the other sites in Chicago I guess. But I don't think anything like Willis or whatever the hell they're trying to call the Amoco building will catch on for now. Chicago has some great architecture, I always love showing outlanders around the place. The Rookery, Monadnock Building, Tribune Tower, Wrigley Building, Carbide and Carbon, Sears Tower, Prudential Building, Amoco Building, etc. These are all landmarks that have been around for many decades. Just putting a new sign on it ain't going to change what I've been saying for thirty years. Course the Amoco is a new name for the building too. Give it another thirty years and maybe enough people will know it as the Aon.