The problem is that it's not clear how useful those terabytes of information are. Ultrasound is very good for certain types of imaging, but the contrast mechanisms available are very limited - super high resolution images of uniform intensity aren't useful. An imaging method isn't useful if it doesn't help you discriminate what you want to see from what you don't. The reason MR is so useful is that it has so many contrasts available (T1, T2, proton density, flow speed, diffusion coefficient, diffusion direction, chemical composition, tissue elasticity, BOLD activation, and many other more esoteric ones). In an hour long scan, even with rapid acquisitions, you usually only get a few gigabytes of data, but that data has a LOT of information about your tissue - that's the reason the scanner keeps starting and stopping and making different noises, it's taking MANY different types of images with complementary information.