The only logic behind it is that Wizards wishes to strongly discourage picking up every piece of mundane trash you can possibly find. Adventurers are supposed to be heroic, and possibly pragmatic, but not so pragmatic that they come back from the dungeon with the copper kitchen sink strapped to their back.
But they DID want a unified rule, so they made 1/5th apply to magic items as well... but here's the catch.
It's specifically based on the idea of a quick sell, or scarcity of money. If you try to get rid of it fast, or reduce it to residiuum, you get 1/5th. But, if you actually take it someplace where there is a demand you can get a more normal price as arbitrated by your DM.