Because this is sadistic torture. Before being released, the bull is forcibly tied down, balls of flammable material are attached to his horns which are lighted and burn the body face and eyes for hours causing immense pain and terrible suffering. The bulls often try to kill themselves to escape the terrible pain. This happens annually in Soria, Spain. There may be rites of culture but this is pure sadistic torture and not acceptable in the 21st century.