It was calculated to upset the older generation that had fought in America’s wars, for the flag was a symbol of all they had fought for. And the police, many of whom were veterans, used to react with violence, which was precisely the reaction the rebels sought to further their cause. Over time, though, U.S. society has become more tolerant of flag-burning.
Because it no longer triggers a reaction, it is no longer used as an instrument to shock. In sum, if group sentiment becomes more tolerant and less easily hurt, the actions that try to hurt it will diminish.