Zstd was a work in progress when Apple released LZFSE even though it was already seeming to be better than LZFSE. But apple needed a finished product and at the time of the release it looked to be the state of the art (released) in a combined metric of compression and performance and they promoted the fact highly. Apple is unlikely to say at the same time that we think a WIP compression scheme would be better than our in a few months or six months later say that please don't use our state of the art compression format anymore its been superseded even if its true specially since its much better than zlib the one people have used for so long.

I think once Zstd gets a bit more mainstream the best they would do is add it as an option.