Why can't I use the null propagation operator in lambda expressions?

using (ProfitCalculatorDataBaseContext db = new ProfitCalculatorDataBaseContext())
{
    var customers = db.Customers.ToList();
    var orders = db.Orders.Where(o => o.OrderStatus != "Готов")
                         .Select(o => new OrdView
                         {
                             oId = o.Id,
                             oData = o.Data,
                             oCustomersMail = customers.FirstOrDefault(c => c.CustomerId == o.CustomerId)?.Mail ?? "Не найдено",
                             oStartPoint = o.StartPoint,
                             oFinalPoint = o.FinalPoint,
                             oTrackNumber = o.TrackNumber,
                             oOrderStatus = o.OrderStatus,
                             oComment = o.Comment,
                             oMoneyPerOrder = o.MoneyPerOrder
                         })
                         .ToList();

    activeOrdersGrid.ItemsSource = orders;
}

using (ProfitCalculatorDataBaseContext db = new ProfitCalculatorDataBaseContext())
{
    var customers = db.Customers.ToList();
    var orders = db.Orders.Where(o => o.OrderStatus != "Готов")
                         .Select(o => new OrdView
                         {
                             oId = o.Id,
                             oData = o.Data,
                             oCustomersMail = customers.FirstOrDefault(c => c.CustomerId == o.CustomerId)?.Mail ?? "Не найдено",
                             oStartPoint = o.StartPoint,
                             oFinalPoint = o.FinalPoint,
                             oTrackNumber = o.TrackNumber,
                             oOrderStatus = o.OrderStatus,
                             oComment = o.Comment,
                             oMoneyPerOrder = o.MoneyPerOrder
                         })
                         .ToList();

    activeOrdersGrid.ItemsSource = orders;
}