If creating new molds is expensive, why Lego sometimes uses new pieces only for 1 set or 1 wave?

I always hear that creating new molds is expensive and Lego wants to maximize their use whenever they can, then why sometimes they only use new pieces for 1 set or wave, even though they could be used more? I want to give the examples of Ninjago, since I'm familiar with it, Sora's current suit uses a paldron with a dragon head on one side, they used the right piece at first, but then this year's waves she has a half-mask and sword holder, despite in the show she keeps using paldrons with the...