Hunter's Mark is closer to Hunter's Quarry in mechanics (assign it to a single opponent), and is available to all Rangers, while only the Colossus Slayer feature offers the extra damage we've come to associate with Favored Enemy. They're both there to fill the same space, but with different "opportunity" costs (a spell, or a feature).
Getting at-will Hunter's Mark at level 2, advantage/resistance to poison at level 5 (= Protection from Poison), an "area" attack with bow/crossbow at level 9 (= Conjure Barrage), immunity to difficult terrain and movement-reducing spells (= Freedom of Movement) at level 13, and "two ranged attacks with bonus action" stance (= Swift Quiver) at level 17, is enough to fill out a "spell-less" Ranger's repertoire. And this can be summed up in a single sidebar.