Like everyone else has said, it matches the control movements of a full scale aircraft. Instead of thinkign about it as moving the stick down to make the plane go up, think about what the plane is actually doing. The plane basically changes pitch. Moving the control stick to the rear, makes it pitch up or rotate to the rear (nose up, tail down) and moving the stick forward has the opposite affect. Also, there has to be standards. Look at cars. You can hop into any make of car sold here in the US and find a familiar steering wheel, common brake pedal location, etc. Someone could dream up countless ways to control a car but this sort of standardization makes things safer and easier to learn. Also, the "standard" has usually evolved over the years to something that works well for the largest group of people.