Well they already did a Kyber crystal story in Rebels, that might be why.I still wonder why they chose to complete "The Bad Batch" but not "Crystal Crisis on Utapau." I mean, "Crisis" is kind of important to the arc, since it's implicitly a harbinger of the creation of the Death Star (Dooku trying to get hold of a huge Kyber crystal to power an unspecified superweapon), and foreshadows Rogue One in a way as well. I'd think they would've wanted to finish it too, given the option.