There was a constraint - since 2009, the Joint Photographic Experts Group had published JPEG XR, JPEG XT and JPEG XS, and they were probably reluctant to break that naming scheme.
They're running out of good options, but I hope they stick with it long enough to release "JPEG XP" :-)
They're running out of good options, but I hope they stick with it long enough to release "JPEG XP" :-)