It was a relatively simple way of trying to address one of the traditional complaints about the Wizard, which was they were too powerful and had "too much utility" (and you can go find that Wizard+ thread right now to see plenty of people still ranting about that.) Having more high-level magic prepared facilitated that, and this new set of rules curtails it a bit.
It also addresses another situation, which was the newly added idea that all classes can cast any ritual spell they have prepared as a ritual. With most rituals being 1st level, having characters forced to prepare more 1st level spells means more ritual spells get prepared, meaning more ritual spells actually being able to be cast as rituals. The 2014 book had the issue that all the classes with Ritual Casting (besides the Wizard) had to have their ritual spells prepared in order to then cast them as rituals... but if you were a higher-level caster how often did your Clerics or Druids and the like actually prepare those 1st level spells for that function? Seems to me everyone tended to only prepare the highest end 1st-levle spells (like Bless and the like)... and the ritual spells got ignored. Which meant that Ritual Caster feature was essentially useless for a lot of PCs. At least with this new system with you now forced to prepare more 1st level spells... you might as well throw in a couple ritual spells so that you can cast them as rituals later on and save yourself the spell slot.