Many reports and analysis seem to be suggesting that losing ZFS helped to shrink Snow Leopard’s footprint (by 6GB). There are also musings that the merger *cough* of Sun and Oracle may have – in some subtle way – reset the (shaky) terms of Apple’s project to port the file system.
We might suppose that ZFS is useful in instances of very large server storage configurations and most Mac servers aren’t massive in storage terms, running quite satisfactorily on RAIDed drive arrays.
Mac desktops and notebooks don’t need it at all, using generally one or two directly attached drives inside or outside the enclosure. Ergo Apple can dump ZFS as not being essential.