Instead of the GVR, the Supreme Court could have heard arguments and ruled on the merits. It did not. I have my guesses why, but they are only guesses. What happened next, to the surprise of absolutely no one, the same District Court judge then struck down the law a second time. The 9th circuit then granted a stay and elected to rehear the case en banc rather than going through the usual 3 judge panel. Oral arguments took place in March. Since then, Bonta has filed Notices of Supplemental Authority for Rahimi, DSSA, and Bianchi and the plaintiffs have responded.
Sometime later this year we can expect an opinion from the 9th circuit, where upon the losing side will once again petition the Supreme Court. But as we saw with Illinois cases in July, a grant of cert is by no means certain. My guess is that until there's a conflict in the circuits, they will continue to do so.