[zinekeller]

Because Facebook doesn't have a browser.

(But seriously, Mozilla engineers have warned the Chrome team that they are too rush with the inclusion of Brotli, since that compression wars are heating up. They still proceeded though, which is unsurprising.)

[lifthrasiir]

While that might well be one reason, it should be also noted that Zstandard optimizes for the decompression speed with a reasonable compression ratio while Brotli concentrates on the compression ratio at the slight expense of speed (though it is very hard to do a fair comparison). This is evident from their defaults, where zstd uses a fairly low level (3 out of -7..22) and Brotli uses the maximum level (11 out of 1..11). But both have the decompression speeds far exceeding 100 MB/s which is the practical limit for most Internet users, so zstd's higher decompression speed wouldn't matter much in the web context.