MC Movie Ghast Piglin Jockey Datatpack

Hi so in the latest snapshot they made it so you can leash mobs with other mobs so I had this idea of recreating the movie ghasts with piglins riding it and I used commands to spawn a piglin riding a bamboo raft and a ghast would get leashed to the raft and it work and now I'm trying to make a simple datapack where 1 in 4 ghasts that spawn in the nether would get replaced by this combo of leashed mobs. 
I've made a simple datapack but it just won't work, anyone know what the problem is? Here's the zip of my datapack