can't add customMeshDefinition to vanilla item

I know 1.8 is veeeeeerrrrrryyyy old, but I wanted to make only a little mod, that coul'd use on public servers.

I want to change the item model based on the displayName, so I tried to add a customMeshDefinition to a diamond_sword, but this doesn't work, so I also added a customMeshDefinition to a custom Item (what I don't want), but this works.

Now I wan't to know  why I can't  add a customMeshDefinition to a vanilla item. And If it is yet possible, pls tell me how

And sory that I still use 1.8

Megabaum