This is because the image is acquired with large gaps between image slices. This reduces acquisition time (therefore reduces cost, chances of motion artifacts, increases patient comfort, and allows acquiring more images with different acquisition protocols), and if the slices are reviewed in 2D then it does not have impact the image interpretation. However, the big limitation is that such images are not well suited for 3D analysis or reformatted views. Often these sparse acquisitions are created in 3 different orientation, which mostly solves the reformatted view problem, but still not offer high-resolution 3D views. See more information here: Combining volumes - what am I missing? - #2 by lassoan