brotli is ubiquitous because Google recommends it. While Deflate definitely sucks and is old, Google ships brotli in Chrome, and since Chrome is the de facto default platform nowadays, I'd imagine it was chosen because it was the lowest-effort lift.

Nevertheless, I expect this to be JBIG2 all over again: almost nobody will use this because we've got decades of devices and software in the wild that can't, and 20% filesize savings is pointless if your destination can't read the damn thing.