As already mentioned, it's mostly due to the bad form they use, which is usually because they do circuits of things like pullups and heavy deadlifts, after they've already done 200 jumps with a rope, trying to beat a time rather than focusing on quality reps. Crossfit style of training isn't necessarily bad if you use good form and to be honest, crossfit athletes usually look in great shape. I can see why this style of training might be appealing to some people, but if your goal is to make optimal strength / size gains in the safest way possible, so you can keep lifting for a long time, crossfit isn't it. The risk of injury is just too much consdering the reward is not higher.