Set selected value for DropDownListFor in controller does not work in razor view

I have this in my controller:

[Route("Checkout")]
public ActionResult Checkout()
{
    var sb = new ShoppingBagViewModel();
    sb.DestinationCountry = "Netherlands"; // I hoped that this was sufficient
    sb.CountriesSl.Single(c => c.Text.Equals("Netherlands", StringComparison.InvariantCultureIgnoreCase)).Selected = true;
    return View(sb);
}

Quick watch in Visual Studio confirmed that the CountriesSl (which is of type List<SelectListItem>) has a selected value. (netherlands)

This is my razor view:

@Html.DropDownListFor(m => m.DestinationCountry, Model.CountriesSl)

DestinationCountry is also a string prop in my viewmodel. I know that there are a lot of similar questions, but I have looked at a lot of them and I just do not see it. I also tried:

@Html.DropDownListFor(m => Model.DestinationCountry, Model.CountriesSl)

Please do not just give me the answer, but also explain what my mistake is.

edit to make clear what the problem is: I get a nice option list in my razor view, but there is no item "selected" just the first one. When I look at the generated html, there is no selected item too.

edit for Ric

public List<SelectListItem> CountriesSl
{
    get
    {
        List<SelectListItem> _countries = HttpContext.Current.Cache["slCountries"] as List<SelectListItem>;
        if (_countries == null)
        {
            _countries = new List<SelectListItem>();
            foreach (string s in System.IO.File.ReadLines(System.Web.Hosting.HostingEnvironment.MapPath("~/tmpwrite/countries.csv")))
            {
                var tmp = s.Split(";".ToCharArray(), StringSplitOptions.RemoveEmptyEntries);
                _countries.Add(new SelectListItem() { Text = tmp[1], Value = tmp[0], Selected = false });
            }
            HttpContext.Current.Cache.Add("slCountries", _countries.OrderBy(c => c.Text).ToList(), null, System.Web.Caching.Cache.NoAbsoluteExpiration, new TimeSpan(24, 0, 0), System.Web.Caching.CacheItemPriority.Normal, null);
        }
        return _countries;
    }
}