There's another big reason to have the 20% rule.
It actually makes treasure, well, treasure.
In pre 3E, if a DM gives out a random magic item, without the ability to buy magic items, it made little sense to sell it since you wouldn't get what you want.
Since 3e, with magic items being available for sale, any magic item that you find, there's a reason now to sell it. However, at a 50% rate, it means that the random treasure makes no sense.
At 20%, you're more likely to get players to actually keep and use said random magic items.
I think it is a rule that, while gamist, actually understands what will happen in the actual game world unlike the 50% rule.