WPF ScrollViewer: Horizontal scrolling doesn't work

Why won't this support horizontal scrolling?

<ScrollViewer>
    <ItemsControl ItemsSource="{Binding Territories}" HorizontalAlignment="Left">
        <ItemsControl.ItemsPanel>
            <ItemsPanelTemplate>
                <VirtualizingStackPanel CanHorizontallyScroll="True" ScrollViewer.HorizontalScrollBarVisibility="Visible" ScrollViewer.VerticalScrollBarVisibility="Auto" >

                </VirtualizingStackPanel>
            </ItemsPanelTemplate>
        </ItemsControl.ItemsPanel>
        <ItemsControl.ItemTemplate>
            <DataTemplate>
                <Grid >
                    <Grid.RowDefinitions>
                        <RowDefinition Height="Auto"/>
                        <RowDefinition Height="Auto"/>
                    </Grid.RowDefinitions>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="75" />
                        <ColumnDefinition Width="75" />
                        <ColumnDefinition Width="150" />
                        <ColumnDefinition Width="150" />
                        <ColumnDefinition Width="65" />
                        <ColumnDefinition Width="60" />
                        <ColumnDefinition Width="75" />
                        <ColumnDefinition Width="75" />
                    </Grid.ColumnDefinitions>

                    <TextBox Grid.Row="0" Grid.Column="0" Text="{Binding Quantity, StringFormat=N0, ValidatesOnDataErrors=True}" />
                    <TextBox Grid.Row="0" Grid.Column="1" Text="{Binding ProductionNumber, ValidatesOnDataErrors=True}" />

                    <ComboBox Grid.Row="0" Grid.Column="2" SelectedValuePath="PrimaryKey" SelectedValue="{Binding RepKey}" ItemsSource="{Binding RepCanidates}" TextSearch.TextPath="FullName" Margin="4">
                        <ComboBox.