First, the index is very large, and it only ever gets larger over time. I just cloned and compressed the crates.io index (https://github.com/rust-lang/crates.io-index), which resulted in a 58 MB archive (note that I did remember to delete the .git directory).

Second, the index changes very often. Every time anyone ever publishes a new version of a package, that changes the index. For crates.io, this happens hundreds or thousands of times per day.

Third, the index is append-only.

Fourth, the index is extremely frequently requested. Any time the user manually asks for an update, or any time the user adds a new dependency, the local copy of the index needs to be updated.

Putting it all together, since the index is constantly changing and since users will constantly be asking for the latest version, this means that it would be very inefficient to serve the whole thing each time. Instead, a fine-grained solution is more efficient. In the early days of crates.io, this problem was solved by just storing the index in a git repo and letting git take care of fetching new diffs to the index (and the problem of "who pays for hosting" was solved by using Github). Now that the crates.io index is outgrowing this solution, it's moving to a more involved protocol where clients will not have local copies of the full index, but instead will only lazily fetch individual index entries as necessary, which is much faster (especially for fresh installs (including every CI run!)).