Datagrid refuse to refresh and show its data, where data is of type datatable in WPF MVVM

<Grid>
  <DataGrid x:Name="dg" ItemsSource = "{Binding dtSrc}"/>
  <Button x:Name="loaddata" Command="{Binding File_OpenCommand}"/>
</Grid>

//in MainWindow.xaml.cs
public partial class MainWindow : Window
{
  public MainWindow()
  {
     this.DataContex = new MainWindowViewModel;
  }
}

//in MainWindowViewModel.cs
public class MainWindowViewModel
{
  public MainWindowViewModel()
  {
    File_OpenCommand = new DelegateCommand(new Action<object>(File_Open));
  }
  
  public DataTable dtSrc {get;set;}
  public DelegateCommand File_OpenCommand;
  public void File_Open(object obj)
  {
      // here is the problem:
      //dtSrc has data, but DataGrid refuse to refresh and show them.
      dtSrc = LoadData();
  }
}

//in DelegateCommand.cs
public class DelegateCommand : ICommand
{
    readonly Action<object> _execute;
    readonly Predicate<object> _canExecute;

    public DelegateCommand(Action<object> execute)
        : this(execute, null)
    {
    }

    public DelegateCommand(Action<object> execute, Predicate<object> canExecute)
    {
        if (execute == null)
            throw new ArgumentNullException("execute");

        _execute = execute;
        _canExecute = canExecute;
    }

    public void Execute(object parameter)
    {
        _execute(parameter);
    }

    public bool CanExecute(object parameter)
    {
        return _canExecute == null ? true : _canExecute(parameter);
    }

    public event EventHandler CanExecuteChanged
    {
        add { CommandManager.RequerySuggested += value; }
        remove { CommandManager.RequerySuggested -= value; }
    }
}