Why doesn't Đ get flattened to D when Removing Accents/Diacritics

I'm using this method to remove accents from my strings:

static string RemoveAccents(string input)
{
    string normalized = input.Normalize(NormalizationForm.FormKD);
    StringBuilder builder = new StringBuilder();
    foreach (char c in normalized)
    {
        if (char.GetUnicodeCategory(c) !=
        UnicodeCategory.NonSpacingMark)
        {
            builder.Append(c);
        }
    }
    return builder.ToString();
}

but this method leaves đ as đ and doesn't change it to d, even though d is its base char. you can try it with this input string "æøåáâăäĺćçčéęëěíîďđńňóôőöřůúűüýţ"

What's so special in letter đ?