Why didn't the big bang produce heavy elements?

Hi Naked Scientists, I downloaded every podcast you guys have put out and have been listening to 2 episodes each day every day at work for months! I'm currently through September of 2009, so I might not hear the answer to this question for quite some time, but this is what I was wondering...

According to the model I looked at, the big bang produced giant clouds comprised of almost exclusively hydrogen and helium, and from these clouds formed galaxies and the other celestial objects. I know that heavy metals are formed when a sufficiently large star goes supernova because of the intense heat and pressure which is necessary to form these elements. Here's my question. The big bang had to have been the hottest and densest of any explosion in the history of the universe, so why did it form primarily hydrogen and helium instead of instantly fusing most of these elements together to form heavy metals?
Nathaniel S Toothaker
Lewiston, ME