That just because if you had gas in the middle it would be uncomfortable for long drives.
It's the same reason the clutch is on the side as well, not in them middle.
I suppose you could swap the cluch and gas (leaving brake in the middle) - but clutch comes first, then gas, so the convention of first on the left is realized.
There was a Top Gear episode that looked at the history of the location of the pedals, and tried to find the first auto that laid them out the way we are used to today.
What was shocking is the enormous variety before then. Just about any of our familiar controls (including some others before we really got a good transmission system) could be on any of your 4 limbs.
Possible, but more likely more people are right footed than left so it's more natural to control acceleration with your right foot vs. the more mechanical shift process.
And I personally prefer Cancel/OK because I usually want to click OK and my right thumb happens to be on the right side of the screen.