I don't really care about space all that much, but packages I build tend to get uploaded and the people downloading them may not have fast internet. Meanwhile, packages built by my AUR helper I care about speed (seriously, it takes ages to compress then immediately decompress firefox). The problem isn't that I want to optimize for one or the other, it's that AUR helpers generally have a different need than I do when building my packages myself, but for some reason AUR helpers don't override the compression setting for just their install. Probably due to caching like I said which means they can't assume everyone will want compression off all the time, but I'm not sure, that's just a guess.

I doubt anybody today has legitimately enough reason to care about space to prefer xz over zstd. The only plausible reason is that some customer, or customer's customer, is only equipped to unpack archaic formats.