Lawn darts were banned in regions like the United States primarily due to the severe safety hazards they presented. The United States Consumer Product Safety Commission (CPSC) banned them in 1988 because they posed a significant risk of causing skull puncture injuries during normal use or misuse. Tragically, the move to ban lawn darts came after incidents, including the death of a child in 1987, which highlighted their inherent danger source.
For more detailed history, you can visit this article on Mashable.