in your example, the QT alert alerted you any time the price is below the set price.
In MT, the crossing alert only alerts you if MT sees the price cross. However, in your example, assuming MT had a quote on the stock at $25, when you open MT when the price is at $21 it should still alert you since it would see the price cross (from the old quote of $25 to $21). If you then shut down MT and open it again there will no longer be an alert however.
We do have the simple price alert on the todo list.