Well, I'm assuming it has something to do with the fact that normal nucleosynthesis processes can't make elements past iron (nuclear fusion consumes rather than releases energy for heavier elements than iron, so the heavier elements are made in supernova or more energetic processes)
I really don't know for sure as I am not knowledgeable about astrophysics, but it sounds a reasonable conjecture, which will help you latch on when you hear the official explanation which I hope will come here.
As this forum is for chemistry and there isn't one specifically for astronomy homework, perhaps not everyone here knows that when astronomers say 'metals' they mean every element that is not H or He.
I just thought you might like to know. :)