My two guess as to why the donphans came out early are:
1. They’re the first ones encountered in the video game as part of Arven’s story dealing with the 5 titan pokemon. 3 now have an ex (the donphans, Klawf, and Bombirdier), so we may get an ex of Tatsugiri and Orthworm down the line.
2. Design process change, maybe they were just going to be regular ex Pokemon until they decided to switch it up and give them special mechanics/cards supporting them since they’re the most “unique” aspect of this last gen. You can’t really do something like Team Plasma specific cards because the “evil team” in this game is technically just a bunch of students playing hooky in a wannabe gang, and there’s nothing like the fighting island in swsh to build off of either.