Adobe in their infinite wisdom did not create a standard for XMP files for non-proprietary image files (JPEG, TIFF, DNG). For those files, Adobe assumes that the XMP data will be written inside the original file. Modifying the original file is a non-starter for many photographers, who want their originals to be pristine and there is also always the chance of data loss when writing to a file. Nitro supports the idea of immutable originals.
Why does Nitro not use “original.xmp” in that case? Because of RAW+JPEG. In that case, there are two original files and both would try to modify the same XMP file, causing a collision / conflict. For example, it would be easy to rate the RAW and flag the JPEG (say to mark it for deletion). But in that case, both images would be rated and flagged with the same value as there is only one standard place to store that metadata.
At some point (hopefully not long, but there are always so many requests for features and improvements), I will offer the ability to write into the original file and also to let people use the “original.xmp” for JPEGs. As mentioned, both of these have some degree of danger. And, it is virtually certain that someone will have data loss either due to some corruption, or because they have R+J and don’t understand that “original.xmp” is completely unsupported for that style of shooting.
The approach I have taken is less compatible with other apps, but is by far the most safe, and data safety is my first and foremost priority.