why does this achievement not work but the other one does?

this achievement doesnt work: 
  "display": {
    "icon": {
      "id": "minecraft:skeleton_horse_spawn_egg"
    },
    "title": "Ghost rider",
    "description": "Ride both undead horses",
    "frame": "goal"
  },
  "parent": "mounts_improved:achievements/broncobuster_achievement",
  "criteria": {
    "ride_zombie_horse": {
      "trigger": "minecraft:started_riding",
      "conditions": {
        "player": [
          {
            "condition": "minecraft:entity_properties",
            "entity": "this",
            "predicate": {
              "type": [
                "minecraft:zombie_horse"
              ]
            }
          }
        ]
      }
    },
    "ride_skeleton_horse": {
      "trigger": "minecraft:started_riding",
      "conditions": {
        "player": [
          {
            "condition": "minecraft:entity_properties",
            "entity": "this",
            "predicate": {
              "type": [
                "minecraft:skeleton_horse"
              ]
            }
          }
        ]
      }
    }
  },
  "requirements": [
    [
      "ride_skeleton_horse"
    ],
    [
      "ride_zombie_horse"
    ]
  ],
  "rewards": {
    "experience": 250
  }
}```

but this one does work:
```{
  "display": {
    "icon": {
      "id": "minecraft:camel_spawn_egg"
    },
    "title": "Cameleer",
    "description": "Ride a camel",
    "background": "minecraft:block/coarse_dirt",
    "frame": "goal"
  },
  "parent": "mounts_improved:achievements/broncobuster_achievement",
  "criteria": {
    "ride_camel": {
      "trigger": "minecraft:started_riding",
      "conditions": {
        "player": [
          {
            "condition": "minecraft:entity_properties",
            "entity": "this",
            "predicate": {
              "type": [
                "minecraft:camel"
              ]
            }
          }
        ]
      }
    }
  },
  "requirements": [
    [
      "ride_camel"
    ]
  ],
  "rewards": {
    "experience": 100
  }
}```