Brandon: It's theoretically possible for Atium to appear in the future, but right now Sazed has no plans to release any of it to the people. It is, effectively, now something of myth and legend.
I also believe it is said somewhere - in the Mistborn annotations, perhaps - that Sazed changed the Allomantic powers back to their original state. I'm too lazy to find the quote, though. Anyways, really, atium is a tool for killing, which wouldn't really be useful when you're trying to rebuild society literally from the ground up. It's just not like Sazed to hand mankind a weapon when they need to be working together.
Brandon: Preservation wanted Atium and Malatium to be of use to the people, as he recognized that it would be a very powerful tool--and that using it up could help defeat Ruin. But he also recognized that sixteen was a mythological important number, and felt it would make the best sign for his followers. So he took out the most unlikely (difficult to make and use) metals for his sign to his followers.
All Brandon quotes were pulled from the Brandonothology.