If they are going back to the literal "Four Crystals" plot device (as opposed to just alluding to it), maybe they simply wanted to bring it back for whatever reason? Like they wanted to see if they could do that storyline in the series' current look, or they just felt it had been gone long enough, and it was time to revisit it? Or maybe the decision was simply fuelled by nostalgia, be it targeting the fans', appealing to their own, or both?