What in particular about a joints structure makes it so vulnerable for injury in hockey?

Nothing: The problem is not with the joint. The problem is with the violence of the contact. Too a large degree human joints ate designed for quick response to avoid trauma not tolerate or resist it. Hockey is a little more forgiving as skates slide as opposed to cleats that don't. Sports that use cleats tend to have higher injury rates.

Type: The type of joint whether it be a ball and socket or hinge joint. Hinge joints are generally less stable eg a knee vs a hip which is a ball and socket joint.