Major genealogy programs tend to be commercial programs. They each choose to use their own proprietary format for their own database that matches their internal data structure.
Their external interface (i.e. input from and output to) other programs is simply to allow data transfer. The GEDCOM standard has been around for a long time, and as imperfect as it is, it has been adopted by 99% of all programs, including Gramps.
Gramps XML is their own format they developed to transfer data between Gramps users in a nearly lossless way. It does this because it mimics the internal data structure of Gramps.
Other programs would not adopt this format for three reasons:
The format mimics Gramps data structure, but would be quite different from other programs data structures and require mapping to and from it by the other program. As a result of this mapping required, it loses its idempotency.
Gramps XML is not an industry standard. Gramps XML is used by Gramps and a few Gramps utilities and PhpGedView can write but not read it (see Programs that support Gramps XML). No third-party genealogy editor provides full support for Gramps XML. Chicken and egg. If other programs used it, more would join.
The mapping would not be an easy thing to implement. To do so, it would need to be worthwhile to do. For a commercial program, it is unlikely that they'll get many users of a free open-source program to convert to their paid program, so the cost/benefit is not there.