Just had this idea pop up in my mind. Instead of relying on volunteers mirroring package repositories all around the world, why not utilise BitTorrent protocol to move at the very least some some load unto the users and thus increase download speeds as well as decrease latency?
BitTorrent would likely increase latency, not lower it. The bit torrent protocol is very inefficient for small files and large numbers of files (https://wiki.debian.org/DebTorrent - see “Problems”).
But I think your question is more “why not use p2p to download files” for which I think the answer is likely “because they don’t need to.” It would add complication and overhead to maintain. An FTP/HTTP server is pretty simple to setup / maintain and the tools already exist to maintain them. You can use round-robin DNS to gain some redundancy and a bit of load spread without much effort either.
Bittorrent is nice for getting isos, but it would pul my hair out if I tried to download patches with it.