I don't know why all the archive sites don't share backups. The Wayback Machine and archive.is are the largest archive sites by far, and they don't share bulk downloads of the majority of the websites they catalog.
They of course don't have to, but having something like Anna's Archive but for website history would be great.
My guess is the sheer amount of data that archive.org has, which means:
- even higher costs associated with seeding archives (egress traffic, storage iops capacity required etc)
- chances of finding a 3rd-party seed for arbitrary file would be pretty slim, which means seeding on your own most of the time, which would make this hardly any better than offering files over HTTP only.
