I got to speak to a couple different Apple file system engineers many years ago over the course of many WWDCs. I think it was around the Leopard or Snow Leopard time frame (2007-2009), where we casually discussed some of the challenges Apple was having with their ZFS effort.

(We didn't talk about iPhone, so either it had not been announced yet, or development was still so closed down, that it never came up as a topic of consideration.)

My recollection was with their ZFS work, while it was clear to everybody that Time Machine should be a big benefactor of ZFS, they were still unhappy with the high RAM requirements (and also not thrilled about the high CPU requirements). Since laptops were then the bulk of their sales, and these machines shipped with much more constrained specs, there was a lot of uncertainty about when/if they were going to make ZFS the default file system.

(Looking it up, the base 2006 Macbook had 512 MB of RAM, and shared RAM with the Intel GMA 950 GPU.)

I recall there always was a secondary backdrop of concerns with the ZFS license and also the fate of Sun, but the engineers I spoke with weren't involved in those parts of the decision making process.