Consider the complexities of APT: package substitutions, virtual packages, provides, requires, recommends, suggests, replacements and patches as pinning and... There's just more complexity involved. Compare the SysV init system, or systemd, or upstart, to the BSD init system that Arch uses. Now, I'm not saying that Arch is better --- something like Systemd has many more features than what it replaces, and in lots of ways it is better. But it is indeed a more complex system.