This is not a facetious question - I've been curious about this, but all the sources I've found have been speculative.
Before the modern game controller, if you had directional controls, they were on the right hand side. On my old typewriter, the 'backspace' key was in the bottom right hand corner. The equivalent 'action' button - the line feed/carriage return lever - was on the left hand side. On the Apple II, the arrow keys were in the bottom right - as were the two arrow keys of the Commodore PET. Video games followed suit - the venerable Atari 2600 controller had the joystick in your right hand, and your left hand working the fire button.
Then, the NES control pad came out. Nintendo patented the design of the DPad and the controller in general, but that didn't stop Sega from copying it in the Master System.
When we had been 'trained' to control movement with our right/dominant hands, who made the decision to put the movement controls on the left hand side, and what were the reasons for it?
American Game manufacturers in the late 70's/early 80's identified early on that most people would use their right hand for controlling the joystick and the use their left hand for any sort of action button. Now on most early controllers, especially the Atari (easily one of the most popular home consoles), there was only one button usually in the top left, with a huge joystick in the middle. This fit the games of the time which usually didn't require much beyond a joystick and maybe the occasional button press; think pac-man or space invaders. There were exceptions of course. The early pong consoles in the late 70's had dials that needed to be turned. There were also early consoles like the Mattel Intellivision, Colecovision and the Ataris 5200 that had the joystick at the top and some weird keypad on the rest of the controller. However, none of these consoles were really "popular" at least in comparison to the Atari 2600, so none of these new controller models caught on.
I'm sure most people know of the infamous video game crash that led to the rise of Nintendo, this is where the famous D-Pad on the left came about. Before I get to that you have to understand something (bear with me here), both hands work in "unison" when doing an action like writing or playing a video game. The right hand is generally considered better for timed and reaction movements where as the left hand is considered better for broad, contextual movements.
The famous Nintendo controller was designed in part by Gunpei Yokoi, who was responsible for the d-pad being on the left. Japanese games were different from American games at the time in that Japanese games tended to be much faster paced and had much more "action", so Japanese arcades tended to have joysticks on the left while having buttons on the right(the opposite was true in America). The thought process was since games like Mario or Contra required more button pressing as opposed to say an Atari 2600 game. The buttons should be on the right since it will allow right handed people to react faster and to mash buttons faster.
Now the actual d-pad came about because Nintendo wanted a cheap system, so they wanted to cut down on costs as much as possible. Gunpei Yokoi saw that a controller could be smaller (and therefore cheaper) by getting rid of bulky joysticks and implementing a small “cross-shaped, thumb-operated, micro-switched lever capable of moving in four directions and addressing up to eight" otherwise known as a d-pad. Nintendo's popularity and success made having the d-pad on the left become the standard in video game production.
This Stanford look into the history of video game controllers is a good read. http://www.stanford.edu/group/htgg/sts145papers/wlu_2003_1.pdf
Also, on the NES controller, why is B to the left of A?
I'm curious, did any of the game designers take ideas from ww2 fighter planes for design cues? I'm an aircraft mechanic so I tend to look close at those type of those things. Like in the P-51 Mustang the left hand would have been on the the stick while the right hand would be on the balls (throttle and mixture).
You might get a more meaningful response in a subreddit like /r/truegaming, although it is an interesting question.
Why is it called a d-pad? I know the early controllers had a and b buttons, did they also have a c button? Or was it named after the orbital symmetry (d-wave symmetry is generally fourfold)?
I don't know if this helps but over at Engadget there's a video that..."Over the course of roughly 90 minutes, Shuhei Yoshida and Mark Cerney cover everything from the former getting banned from Nintendo's Miiverse (twice), how the PS Move controller signaled a new era of design teamwork at Sony and what it was like working under SCEA's legendarily hard-nosed chief, Ken Kutaragi."