Why did they stop selling clackers?

Clackers were taken off the market in the United States and Canada when reports came out of children becoming injured while playing with them. Fairly heavy and fast-moving, and made of hard acrylic plastic, the balls would occasionally shatter upon striking each other.

Why are clackers dangerous?

The toys could be made out of wood or metal. The ones made of hard acrylic plastic, however, could shatter on impact and become shrapnel. In the US, regulating the safety of toys was originally the job of the Food and Drug Administration.