I think it has to do with more area in the tail, so it will trap and hold better whilst nose riding..
Square tails tend to stall and turn whereas a pin will tend to turn in an arc, the square tail if stepped on hard will stall and turn, which is good if you want to stall and get back up to the nose.
Maybe... Possibly, no idea, just of the top of my head...
I once heard someone say the drop knee turn was never about style it was how you got those old heavy square tails to turn, where as the hard rail all rounder or HP surf better when turned like a shortie IMO..