Not an expert either by far, but DICOM isn't just images, it's a much larger data structure and following protocol, which deals with the transmission of the data between devices as well. The files are just one part of it. It also supports more dimensions to the data frames other than 2D. The pixel data in DICOM itself uses JPEG however (among other potential pixel compression methods), and it is possible that JPEG XL rolled into DICOM could allow for benefits.
Lots of metadata about the image scan. Patient name/id, etc. and often physics related to the image scan such as the voxel size, units, properties of the imaging equipment, etc. DICOM is also used for other data that isn’t directly related to the visual image such as segmentation of the image, radiotherapy planning info, and so on. There are hundreds of dicom tags (the metadata entries) that can be in the dataset.