If you're looking at high-end cyclocross bikes, chances are you're looking at carbon fiber. Carbon has a few advantages over other common materials, such as superior strength-to-weight ratio, longer fatigue life, and increased vibration-damping. While they may cost a bit more, ask any serious cyclist, and they'll tell you carbon's worth it.
Aluminum's main advantage as a frame material is cost. Aluminum frames are less expensive to make. This means they are less expensive to buy. Aluminum frames are also stiffer than carbon or titanium. This can give the rider a speed advantage during short sprints, but the added vibration can fatigue the rider on longer rides.