Pocket were added as it were easier than implementing their own Reading List, although Pocket will be shipped as a default extension in the future rather than being integrated. https://bugzilla.mozilla.org/show_bug.cgi?id=1215694
You sure you're not confusing Reading List and Reader View? Work on Reader View is still happening in bugs such as this one: https://bugzilla.mozilla.org/show_bug.cgi?id=1175536