[csswizardry]

My guess, as someone who doesn’t know the answer but works closely in this field, is that the potential downside (i.e. privacy/security) was much greater than the actualised benefit (i.e. performance).

Safari (who make a big deal about being privacy-conscious) was the first to introduce cache partitioning, looking into it as far back as 2013[1]; Chrome followed in 2020 and Firefox in 2021[3]. One thing I know, anecdotally, to be a strong motivator among browser vendors is ‘X is doing it, why aren’t we?’

1. https://bugs.webkit.org/show_bug.cgi?id=110269

2. https://developer.chrome.com/blog/http-cache-partitioning

3. https://blog.mozilla.org/security/2021/01/26/supercookie-pro...