Back in the early 90s, WASD was sitting the furthest from the arrow keys. Me and a friend would both sit behind the computer, playing a multiplayer game (Wacky Wheels, at least). We could sit next to each other. The person with WASD was at disadvantage because we were used to arrow keys, and the person who sat behind arrow keys had an easier view on the (CRT) screen and far our small hands the WASD were further away from the table, so the right side was all in all way more comfortable. But there was one caveat. To drift, you'd push space bar, and you could smash that space bar to make the other person go drift, meaning (it did a 90 degrees) you could time your shot with it. I'm sure with single player, space bar was used to drift. So I assume it was also like that in multiplayer, but I am not sure for which player it was set up. Because some games changed the layout in such a way that the keys of one player were more near each other, and the keys of the other player as well. Instead of merely adding the other keybinds, they'd say OK you have insert and del as keybinds instead of ctrl and alt. That type of stuff.
Later on, we got a mouse (or, well: to be honest I already had one but it wasn't used as much as in later days), and the mouse was easiest to use in the right hand. So in the Windows 9x era (or well, even DOS, as Dune 2, C&C, WC2 I played in MSDOS) the mouse would be at the right side. And then you would use keybinds / choring with the left hand mostly. But the right side was using the mouse. Especially in 3D FPS this would turn out to be ace (such as Quake 2). Plus, also, the most weight on typing is on left qwerty hand. So the movement with one hand typing while keeping using mouse (small amounts of typing) is easiest from left hand being on left side of qwerty keyboard.