Tying the spells to slots on a per level basis is an absurd constraint that adds nothing of value and boils down to saving them a column for "total spells prepared" on the character level grid.
Just a nitpick: “total spells prepared” isn’t a thing on the class feature grid for casters who memorize, like clerics and wizards. Plus, it’s not a consistent number since it’s modified by caster stat bonus. So, that’s not the reasoning.
I think it might be in part a power balancing thing. It might also open up a bit of design space for “downcasting” spells.
I also think it would be really interesting if this spell prep applied to all the classes that do do prep,
except the wizard, who could keep the current 5e version.
Also, this isn’t set in stone. It’s a playtest doc. Tell them you don’t like it, and if it gets less than that 70% (or whatever the number was) positive rating, then it won’t see the light of day.
For the record, I’m not a fan of the change either, but having played around 30 years with Vancian magic as part of the game, it’s not a dealbreaker if they keep it.