The kick hit up near the knee which is much stronger than the shin.
But, that alone shouldn't have caused such an injury, otherwise this leg break would be much more common than it actually is.
For me, the main reason should likely be overtraining which resulted in a weakened shinbone.
The psychological trauma will be 100 times worse than the physical damage, but Anderson can always modify his roundhouse technique to strike with the ball of the foot instead of the shin (like in Taekwondo or Karate). I doubt he'll try a low-kick striking with shin ever again.