Both lower leg bones are vulnerable to being broken in that they're close to the surface, and force directly impacts the bone. There's not a lot of protective soft tissues around them. Ten to 15 percent of all lower-leg fractures are open fractures of the tibia, in which the bone protrudes through the skin. As you might imagine, an open fracture requires a lot of force. It carries an added risk of infection, as well.
Football is by far the most common, with skiing probably second.
We also see these lower-leg fractures in baseball and basketball. If it goes on, a stress fracture can eventually develop into a complete break, but that's rare. The athlete would have to be pushing himself hard, and ignoring what his body is telling him, for that progression to occur. The tibia is the main weight-bearing bone of the lower leg, so a tibial fracture is more serious than a broken fibula. There's not much tissue around the tibia, and therefore not as much blood supply to the bone.