If you're the author of a new programming language, it's easier for you to develop a plugin for an existing package manager than to develop a new package manager.
Not that I know of. I haven't come across many that are thoroughly documented for the sake of people that come later and furthermore many come with the baggage of the decisions made early on that are difficult if not impossible to change. No one wants a "so so package manager" that supports plugins.
Furthermore, writing a package manager is usually one of the first large scale projects common to most language ecosystems that really exercise what a language is capable of. Other tools for working with the language, such as parsers, interpreters and compilers are among the first projects in any ecosystem.
