In many cases it's because the standards don't (yet) support storing important information or weren't performant. For example, in DTI imaging recording of the d

In many cases it's because the standards don't (yet) support storing important information or weren't performant. For example, in DTI imaging recording of the diffusion vectors was not something that DICOM supported until DTI use became more widespread clinically. In other cases it was network performance hacks. Another example with DTI is that initially DICOM for MRI only supported 2D images. Which for DTI is a problem because you easily end up with thousands of images and transferring them as individual 2D with all the network handshaking and storage confirmations is slow as molasses and grinds on PACS that aren't setup for this. Some vendors "solved" this by creating large 2D images of 3D images by laying out multiple 2D slices in a mosaic. Because the 3D volume became one 2D image, transfers were much faster and doesn't consume as many resources on PACS. Eventually the DICOM standard came up with an even better solution, but it's only recently starting to roll out on the scanners and the old ways still persist.