There is concern of scope creep of having one content model that is responsible for both WikiProjects and some of the features on WikiProjects themselves. The decision was made to split the one content model we have into:

• WikiProject: intro, table of contents, member list, scope, and various embedded pages. Those embedded pages can be regular wikitext pages, or wikilist pages (see below), or something else altogether in the future.
• Wikilist: structured article lists. They can be manually curated lists of page titles or dynamically generated from category sets or even Wikidata queries, but for now it should probably just be general lists of pages editable by humans and bots.

We want to create a sub-product for lists because we have a lot of them on WikiProjects, of different sources (Wikidata, SuggestBot, Wikipedia Requests tool, manually maintained lists), and having a standard list format can standardize presentations—leading to a more consistent user experience—and can be used in other contexts (as discussed with @TrevorParscal).