These days, aluminum frames are probably the most common material out there for bikes. It is therefore most reasonable to assume that most cracked frames are going to be aluminum, due to the number of them on the road. People also tend to treat the higher priced carbon bikes with more care than they would the cheaper aluminum frames, making it even more likely to see aluminum being broken more often.

I don't subscribed to the theory that aluminum is a weak material for bikes. I see a ton of them on the road with no problems, and I myself have an aluminum bike that I put most of my miles on. I'm also a bigger guy (6'5, 230) and put the bike through a lot of stress with no problems at all through many years of service.