Why is there only a single version of each shared library in the official repositories?
Several distros, such as Debian, have different versions of shared libraries packaged as different packages: libfoo1, libfoo2, libfoo3 and so on. In this way it is possible to have apps compiled against different versions of libfoo installed on the same system. Unlike Debian, Arch is a rolling-release cutting-edge distribution. The most visible trait of a cutting-edge distribution is availability of the latest versions of software in the repositories; in the case of Arch it also means that only the latest versions of all packages are officially supported. By dropping support for outdated software, package maintainers are able to spend more time ensuring the newest versions work as expected. As soon as a new version of a shared library becomes available from upstream, it is added to the repositories and affected packages are rebuilt to utilize the new version.