It's because you're using the custom options object - you need to use the configurable options object to pass through configurable attributes using the "configurable_item_options" object.

    {
      "cartItem": {
        "item_id": 0,
        "sku": "string",
        "qty": 0,
        "name": "string",
        "price": 0,
        "product_type": "string",
        "quote_id": "string",
        "product_option": {
        "extension_attributes": {
           configurable_item_options": [
           {
              "option_id": "string",
              "option_value": 0,
              "extension_attributes": {}
           }
         ]
       }
    },
    "extension_attributes": {}
  }
}