I understand that rustup can be convenient, but I don't get why they insist on using this tool. It should be enough to distribute the toolchain and std as a zip file.
Apart from that rustup makes it easy to add and remove optional components, libraries for cross-compilation, and update Rust itself. These things could in theory be done with apt if Debian repackaged all of these components, but they didn't. Having one way to manage Rust installation that works the same on all platforms makes it easier to teach Rust and provide install instructions for software.