Good question. Wish I had an answer :P. If I had to guess the same reason torrents are what they are right now. I'm not well educated in the inner workings, so grain of salt.

I think blossom has it's toes in a good spot when were talking normie IT hosting space. That is, it plugs right into the existing legacy hosting model (some corporation bares liability and "lightly" censors content to keep them out of jail) has a very simple api, and allows some distribution with replaceable host-names for some amount of distribution. But really I'd say it's ideal for the API and clients. 

That being said, I have ideas for easily publishing build artifacts from build servers and attaching them to git-tags via nip34 (maybe) and I have no idea how difficult that would be to automate via torrent instead.