DropDownListFor Inside EditorFor Not Selecting Values

I have found couple questions about my problem but none of them actually addresses the problem and gives alternative solutions to problem, thats why I am asking this again.

I am using strongly typed HTML Helpers and no ViewData and ViewBag for only page titles.

Here is my problem.

I have following viewmodel.

public class RegisterViewModel
{

    public string Mail                      { get; set; }
    public string Name                      { get; set; }

    public ThreePartDatePickerViewModel ThreePartDateSelection { get; set; }

    public RegisterViewModel()
    {
        ThreePartDateSelection = new ThreePartDatePickerViewModel();
    }
}

Above viewmodel uses below viewmodel which basically holds data for 3 dropdownlist which are Day, Month and Year.

public class ThreePartDatePickerViewModel
{
    public string Day              { get; set; }
    public string Year             { get; set; }
    public string Month            { get; set; }

    public IList<SelectListItem> Years      { get; set; }
    public IList<SelectListItem> Months     { get; set; }
    public IList<SelectListItem> Days       { get; set; }

    public ThreePartDatePickerViewModel()
    {
        var now = DateTime.Now;

        Years = new List<SelectListItem>();
        Months = new List<SelectListItem>();
        Days = new List<SelectListItem>();

        var empty = new SelectListItem { Text = "" };
        Years.Add(empty);
        Months.Add(empty);
        Days.Add(empty);

        foreach(var item in Enumerable.Range(0, 100).Select(x => new SelectListItem { Value = (now.Year - x).ToString(), Text = (now.Year - x).ToString() }))
            Years.Add(item);

        foreach(var item in Enumerable.Range(1, 12).Select(x => new SelectListItem { Value = x.ToString(), Text = CultureInfo.CurrentCulture.DateTimeFormat.GetMonthName(x) }))
            Months.Add(item);

        foreach(var item in Enumerable.Range(1, 31).Select(x => new SelectListItem { Value = x.ToString(), Text = x.ToString() }))
            Days.Add(item);


    }
}

In the action method which returns RegisterViewModel to view, I am setting the Day,Month and Year properties of ThreePartDatePickerViewModel.

I have checked these values on runtime while views are generated and they are correct.

In my main view,

@model Bla.Bla.RegisterViewModel

<!-- Helpers for other properties -->

@Html.EditorFor(m => m.ThreePartDateSelection)

And my ThreePartDatePickerViewModel Editor Template is

@model Bla.Bla.ThreePartDatePickerViewModel

<div class="threePartDatePicker">

    @Html.DropDownListFor(m => m.Day, Model.Days)  
    @Html.DropDownListFor(m => m.Month, Model.Months)
    @Html.DropDownListFor(m => m.Year, Model.Years)

</div>

In my final rendered html, I have all the controls as expected. Dropdowns are rendered fine except the values I have set in action method are not selected.

If I switch from EditorTemplates to Partial Views, It starts working. Or If directly render dropdownlists inside my main view, instead of passing model to EditorFor, It again works.