Parmandur
Book-Friend, he/him
I think looking it from the other direction can shed light: the Spells that get Ritual are still that WotC is very comfortable letting Wizards do without any resource drain in any situation, like the vottom of a vast dungeon complex, as long as they have time to focus.I think it's less about meriting a spell slot to cast and more about not wanting wizards to be able to cast them without having to prepare them.