From source: creates much more work for the user.

Package managers: ecosystem is fragmented, requiring a long list of distro- and package-manager-specific instructions. Many scripts already install through package managers, they simply make the user’s life easier.

Flatpaks: These are clearly designed for desktop applications, with CLIs treated as an afterthought. They may be the best long-term hope, but today they are definitely not as convenient or widely available as a simple script.

If you care about adoption, `curl | sh` is the only real option today, which is why virtually all project show it as the first option.