Was there still a time limit in 2005? Maybe Lucas didn’t want to make a shorter cut again.
As far as I know, they upped the limit from 120 minutes to 167 in time for The Matrix Reloaded’s release in May of 2003, and that was only two minutes shorter than RotS.
It was probably a combination of George wanting more time to work on the 35mm and digital versions of the movie (Imax probably needing more lead time to get the 15/70 prints made and shipped to theaters before release day) and the fact that Batman Begins was coming out in Imax only a month later.