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THis is because to reduce the pressure on the snow so as to move swiftly.This happens as if the cross section are of a substance is high the pressure is low and if the area is small the pressure will be more, as pressureα1/area.
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why long flat ones ? as per your answer they could also be made flat and wide in a large circular or square shape.

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Due to the pressure underneath the skies, the snow gets softer and planar, and gets molten a little bit.  So the pressure needed is just enough to achieve a small layer of water underneath the skies.  Long skies are needed so that when a skier goes at a great speed, the snow at a meter ahead of the person, gets softened for skiing over that by the time the skier reaches that point.

Also, the area under the skies is to be large enough so that the person does not sink into the snow, due to high pressure/stress due to exertion of weight.

It is like ice melting under ice skates, when one skates on ice.

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