Because these type of dogs attract some knobhead owners who get them for "show" and "fighting" ... because of their builds and looks. Having a dog is like having a kid, you've got to raise it properly.
However, dogs should never be left alone with a small child, or any child who isn't of a responsible age/strong enough/etc.
It's sad that this keeps happening, and I still agree with others that a license should be needed to own a dog anyway. It shouldn't be needed, but the fact is, these days, it pretty much is because of things like this.
Sad for the entire family here, especially the kids dad.