Overthinking Black Clover Lore (Again)
I just realized this today but I now know why it's only four-leaf grimoires owned by elves that can become five-leaf grimoires owned by devils.
These things. The magic stones.
They belonged to the elf tribe, as stated by the Witch Queen and as seen when Licht had them on hand during his wedding (weird fashion choice by okay).
But the detail I kept on forgetting is that they're connected to the Underworld.
That connection explains why they were used to cast Forbidden Magic such as Licht's demonic transformation and Zagred's reincarnation of the whole elf tribe. It's also why Secre using the stones incurred the curse of the Weg on her.
But there's just a little more. The monument that the magic stones are put into and then the depiction of the Tree of Qliphoth are based on two representations of the Tree of Life in Kabbalah. You don't need to to any in-depth research to find this out.
The elves and devils are connected through the magic stones and that's why it's only elves who can have their four-leaf grimoires become five-leaf grimoires.