Well, let's do a little quick math (after noting that top line films are released in larger formats than 35 mm)...
For projection on a large screen, from a digital projector (which means the pixels from each frame will be the same place as the last, so they need to be small enough not to be visually obtrusive), we'll need minimum resolution (at 16:9 aspect ratio) of something like 10 megapixels, though standard 4:3 35 mm movie frames, 18x24 mm, are capable of recording something like 30-40 megapixels and it may be possible to see the difference between 10 and 40 MP in projection, as it's often possible to see the difference between 35 mm and 70 mm film in projection (and practically always possible to see the difference between 16 mm and 35 mm).
So, if we allow a minimum of 40 megapixels per frame, and a frame rate (again, for top line theatric projection, not for the 24 fps 16 mm sound-on-film that used to get shown in the high school auditorium for assembly) of (IIRC) 60 fps, we get 2.4 gigapixels per second. Since skies can show significant banding (which is both distracting and looks cheap) in 8 bits per channel, we'll assume 12 bits per channel for color, or 4.5 bytes per pixel -- putting us at 10.8 gigabytes per second for theatrical quality digital projection. Multiply by a common movie length of 6000 seconds (for a short feature) up to 15000 seconds (for a long one, like Return of the King), and we get somewhere between 65 and about 170 terabytes for a feature film.
Storing that kind of data isn't a big deal; an optical jukebox with that kind of capacity was available at least ten years ago. Transferring that rate in real time to the projector, however, is quite another issue; that's about 50 times the fastest burst transfer mode my computer can use between hard disk and RAM, and close to 300 times the best sustained speed (though mine isn't by any means the fastest desktop unit around). Compression can help -- but in theaters you also want to avoid compression artifacts, meaning (generally) either a relatively low compression ratio or lossless compression (which only has low compression, relative to JPEG and MPEG).
The technology to do all this is available -- including the necessary resolution of bright-light tolerant, high speed LCD shutters with optical merging to give the screen size and three color channels -- but at a cost of more than a million dollars a screen, last time I checked, and then you still have to get the data to the theater system. A projector capable of showing 70 mm, 60 fps film is technology at least twenty years old, and stacks of film cans are technology that goes back to the days when George Eastman and Thomas Edison created 35 mm by way of Edison holding up his fingers and saying he needed film "about that wide".