Why was ⌘ used as the command key on Apple computers, why not the Apple logo?
From the Wikipedia entry on the Command Key:


  The "⌘" symbol (the "looped square") was chosen by Susan Kare after Steve Jobs decided that the use of the Apple logo in the menu system (where the keyboard shortcuts are displayed) would be an over-use of the logo.


It is also interesting that the ⌘ is not something Apple came up with. It is actually an ancient symbol called looped square.