why the template ui did not show the vue data

I am a newbie with Vue, now I define the vue code like this:

import Vue from "vue";

export default Vue.extend({
  data: () => {
    message: "show message";
  },
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.17/vue.js"></script>
<template>
  <div id="app">
    <div v-text="message"></div>
  </div>
</template>

why the UI did not show the Vue data message? this is my sandbox version code: https://codesandbox.io/s/loving-sea-snvfj?file=/src/App.vue:149-237