I am going to have to vote Tradition. It's what people have been told, forever, just like so many other "myths" in the firearms world.
First off, the difference in gauge is merely the size of round, cylinder shaped solid lead balls, equal to the inside of the muzzle, to equal one pound. 12 Gauge - 12 round balls. 20 Gauge - 20 round balls. The only difference is when the .410 comes in to play...
Now, I own a 12 Gauge because I really wanted that particular model, the 12 Gauge is prominent, and it's ability to find ammo, or resale, will be easier than something obscure.
There is nothing wrong with a 20 Gauge - If you take a blast in the chest with it, you are probably not getting up. If you do, you won't be going far.
But, modern literature of the past, what, 60 years has people using the 12 Gauge, and that is what the common thought is. Everyone uses a 12 Gauge, so it must be the best.
The gun world is filled with marketing campaigns that turned into legend. Hell, the modern world is full of them. Why the hell do you think every woman has a diamond on their finger when they get married - marketing, marketing, marketing....