Debian wants to switch a modern event-based init system that is more robust and provides more features, provides stable support for advanced environments (e.g. SAN), being more similar to the likes of Ubuntu and RHEL, and modern open-source packages like GNOME 3.x are easier to package. Among other reasons, Debian hasn't been quick to switch init systems over lots of work needing to be accomplished.
In one of the latest init system discussions, it was stated "since the init system strongly shapes many other packages, there has to be only one and no other supported options."
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