Ahah a question that would be answered in many a secondary school physics lesson. There are two effects at work.
1) The inner container in the flask has walls that enclose a vacuum. Since heat cannot travel through a vacuum, heat loss by conduction is minimised for the part of the inner flask that is so enclosed (i.e everything but the stopper, which is itself well insulated).
2) The same inner contained is "silvered", a bit like a mirror, to reduce heat loss by radiation.
