You can only repair any item just 6 times on the anvil before it becomes too expensive; anybody would rather use Mending instead so you don't even need any materials (though I think it should work like renaming did before 1.8, making you have to use materials to repair items on the anvil along with the repair costs depending on the item and enchantments):
In fact, I believe they originally added Mending just for Elytra, with other items able to receive it as a bonus.
I believe Elytra were exempt from this, allowing you to repair them using Leather as many times as you wanted, because of their rarity in comparison to normal tools or armor. This is also the reason Elytra don't disappear when they lose all their durability.
Rollback Post to RevisionRollBack
I'm no longer allowed on this forum. Because I don't have a Twitch account.