Traditional IMAX is shot on 70mm, 15 perf film that is so large they ditch the audio track on the negative and throw it through the camera with air blasters, the result of which though is a picture that's considered the holy grail of cinema - the perspective, the resolution, not to mention the crazy audio setups they put into theatres was nothing short of incredible.
The problem with such a huge negative was that it was insanely expensive to print film for theatric releases. To bridge the gap, IMAX started introducing a replacement digital system, which was pretty damn bad. Think two, 2K projectors creating a dithered image versus the equivalent of a 12K image. It's just not the same. Their new, laser based 4K dithering system is a lot better, but still a huge cry from the original GT 70mm projectors they used in places like Sydney.
TLDR, IMAX film was massive. Huge. Inspiring. But it was expensive, and all the things that made IMAX film so good still haven't been faithfully recreated yet.