Because there is no UE4 importer? (I couldn’t find one following your link) Because there is no working and tested Blender exporter? (the one that says “work in progress” could be forever in that state, know how Blender Foundation “cares” about game dev)
OpenGEX has working exporter for Blender, it’s a plain ascii format, and well documented. Still no importer for UE4.
IQM is exporter, has docs and also can export into binary IQM, which creates smaller file and exports faster as I recall.
Personally I don’t really care about the format as long as it works with both Blender and UE4 and is not an abandoned format (and has nice workflow and docs).