why is Saturn's north pole a hexagon?

Well, ..., I believe you're referring to the polar vortex over Saturn's North pole. The most recent (and reproducible) theory postulates that and eastward flowing jet stream near the North Pole is perturbed slightly by the planet's rotation causing the Hexagonal shape to appear. See the article below.

From Wikipedia:
tne hypothesis, developed at Oxford University, is that the hexagon forms where there is a steep latitudinal gradient in the speed of the atmospheric winds in Saturn's atmosphere.[14] Similar regular shapes were created in the laboratory when a circular tank of liquid was rotated at different speeds at its centre and periphery. The most common shape was six sided, but shapes from three to eight sided were also produced. The shapes form in an area of turbulent flow between the two different rotating fluid bodies with dissimilar speeds

That's simply how rotating fluids behave (gasses and fluids behave the same). It has nothing to do with Saturn. They act like that here too. We've duplicated this in the laboratory. In Saturn's case it's a combination of the jet streams in the atmosphere and the planet's rotation.

http://news.sciencemag.org/2010/04/saturns-strange-hexagon-recreated-lab

It is not.

This is just a guess but Saturn may be made from hexagonal molecules hence everything on Saturn is hexagonal. Saturn is also connected to the idea of Satan who is also represented by the grematia symbol 666 also connected to the shape of the hexagon. Its positioning in the solar system might also be the reason why things are hexagonal on Saturn, causing only hexagonal things to form there.