Simple: Brotli was initially shipped as a part of WOFF2 compressed font file format, and its creators foresaw more possible uses in the web browser, tailoring i

Simple: Brotli was initially shipped as a part of WOFF2 compressed font file format, and its creators foresaw more possible uses in the web browser, tailoring its built-in preset dictionary for web contents. It is no doubt that Zstandard is faster than Brotli, but both are sufficiently fast enough for web contents (100 MB/s and more), so an additional gain from Zstandard was much smaller. Given this, I believe Zstandard support in web browsers was possible only due to the strong push from Meta.