I read David Breashears book High Exposure: An Enduring Passion for Everest and Unforgiving Places which talks quite a bit about the making of Everest (1998) and his path to learning to embrace and ultimately love the IMAX camera. Each reel of blank film stock which goes into the camera is only 60 seconds (or was it 180?) long. That kind of abbreviated length forces the camera man to really concentrate on the subject.
Given the major expense of the camera and the proportion of the camera devoted to handling that huge film, besides tradition and its long proven technology, replacing it with a digital version of the camera would deter filmmakers who have already invested in the equipment (which can no longer be used) and all the downstream work flow services and equipment would be a very hard choice to make.