Не задается значение default для ComboBox

Создадим такую ViewModel

public class MainPageViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    private readonly IMainPage _mainPage;
    //ctor
    public MainPageViewModel(IMainPage mainPage)
    {
        _mainPage = mainPage ?? throw new ArgumentNullException(nameof(mainPage));
    }

    /// <summary>
    /// Содержимое комбобокса
    /// </summary>
    public List<string> StatusList { get; set; } = new List<string> { "Active", "Inactive" };

    /// <summary>
    /// Индекс выбранного в комбобоксе
    /// </summary>
    private int _SelectedIndexStatus;
    public int SelectedIndexStatus
    {
        get { return _SelectedIndexStatus; }
        set
        {
            _SelectedIndexStatus = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(SelectedIndexStatus)));
        }
    }
    /// <summary>
    /// Кнопка ОК
    /// </summary>
    public DelegateCommand ButtonOKCommand => new DelegateCommand(OnButtonOK);
    private void OnButtonOK()
    {
        _mainPage.ShowMessage($"Вы выбрали: {StatusList[SelectedIndexStatus]}");
    }
}

Такой кодбихайнд

public interface IMainPage
{
    void ShowMessage(string message);
}
/// <summary>
/// An empty page that can be used on its own or navigated to within a Frame.
/// </summary>
public sealed partial class MainPage : Page, IMainPage
{
    public MainPage()
    {
        this.InitializeComponent();
        //привязка ViewModel
        var vm = new MainPageViewModel(this);
        this.DataContext = vm;
    }

    public async void ShowMessage(string message)
    {
        var messageDialog = new MessageDialog(message);
        await messageDialog.ShowAsync();
    }
}

XAML такой

<ComboBox HorizontalAlignment="Left"
              Margin="100,0,0,0"
              Grid.Row="1"
              VerticalAlignment="Center"
              Width="152"
              ItemsSource="{Binding StatusList}"
              SelectedIndex="{Binding SelectedIndexStatus, Mode=TwoWay}"/>
    <Button Content="OK"
            Margin="100,0,0,0"
            Grid.Row="2"
            VerticalAlignment="Center"
            Command="{Binding ButtonOKCommand, Mode=OneTime}"/>

            <ComboBox
            Name="StatusAssetClass"
            Background="White"
            Height="45"
            HorizontalAlignment="Stretch"
            Margin="16,10,16,10"
            SelectedIndex="0"
            Style="{StaticResource ValidatableComboBoxStyle}">
            <ComboBoxItem>Active</ComboBoxItem>
            <ComboBoxItem>Inactive</ComboBoxItem>
        </ComboBox>

Данные получаю с ComboBox таким способом:

StatusAssetClass.SelectionBoxItem.ToString()