Why Didn't the Nandas Survive?

The only memories we have of the nandas to day is that they were rich, tyrannical and arrogant oppressors. We've seen it happen through various parts of the world for centuries. And we can see contemporary figures who can also best be described as petty as hell. So i know what i believe personally. It up to you whether you want to believe that historical figures were capable of that kind of behavior.