Because it worked so well that it wasn't needed anymore.
Carrying a gun is a responsibility and a physical impediment. In many states, the crime rate was so low that it just wasn't thought worth while. Add to that:
In the South, it was a plot to disarm black people, knowing that the law would not be applied to white people.
After 1900, it was propaganda by "Progressives" who wanted to disarm the populace.