为什么这个列表框没有垂直滚动条

<UserControl x:Class="SimpleStack.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">
    <UserControl.Resources>
        <DataTemplate x:Key="ListBoxItemTemplate">
            <StackPanel Orientation="Horizontal">
                <TextBlock Text="Place holder"/><TextBlock Text="{Binding}"/>
            </StackPanel>
        </DataTemplate>
    </UserControl.Resources>
    <Grid x:Name="LayoutRoot" Background="Azure">
        <Grid.RowDefinitions>
            <RowDefinition Height="auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="auto"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        <TextBlock Text="The Text" Grid.Row="0" Grid.Column="0" Grid.ColumnSpan="2"/>
        <ListBox ItemsSource="{Binding ListOfNumbers}" Grid.Row="1" Grid.Column="0"
                 ItemTemplate="{StaticResource ListBoxItemTemplate}"/>
        <TextBlock Text="Place Holder" Grid.Row="1" Grid.Column="1"/>
    </Grid>
</UserControl>