Tubing Atlantis frames are built with CrMo steel, mainly because steel is tougher than aluminum and titanium. Toughness is not just important for durability, but for safety, too. A material which is not tough tends to fail catastrophically because a crack in it grows too fast for you to notice it or react to it. Aluminum and titanium fail catastrophically. That doesn't mean they're lousy frame materials.
Engineering can make a big difference in preventing cracks in the worst places.
But if a crack does form, tough steel will slow it down and give you time to get off the bike, or even finish the ride. People have ridden cracked steel frames for months without even knowing it. That is not recommended, but it's better than having your bike fail suddenly beneath you. The seat tubes, down tubes, and chainstays are heat-treated, because they're the only tubes that ever break on any bikes, and we wanted an added margin of security. The top tube and seat stays are standard CrMo steel.