Почему метод выполняется в главном потоке?

public partial class MainWindow : Window
    {

        public async Task<List<String>> getDataa()
        {
            var client = new RestClient("http://url");
            var request = new RestRequest("getTopics/", Method.POST);
            var path = @"D:\Dataset\entertainment_news_article_10355.txt";
            request.AddFile(path, path);
            var response2 = await client.ExecuteTaskAsync<List<String>>(request);
            return response2.Data;
        }
        public MainWindow()
        {
            InitializeComponent();
            BT.Click += async (sender, e) =>
            {
                List<String> res = await getDataa();
                LB.ItemsSource = res;
            };
        }