Never saw it in IMAX but I would venture it was the same reason that it had a two hour limit, in that it was one of the first IMAX conversions ever done. Besides which, it would not surprise me if that was justr the way AOTC was and the IMAX format made it obvious: one of the reasons Lucas liked the not-ready-for-prime-time HD television cameras that they shot the film with was because he liked to blow up the image and do in-frame zooms and pans, which are in a million damn scenes in the film, and he felt HD gave better leeway rather than film. He said he was able to artificially blow up the image and do digital pan and scans 20% more than with film. He was basically nuts in all his opinions regarding digital video photography in the year 2000 when the movie was put to camera. So, without having any real frame of reference, my first instinct is to say it was just the movie itself. For anyone who worked in cinematography (eg me) AOTC is a complete clusterfuck, but then a lot of the movie is like that.