Why most of public figures lost their devotion in their cause when get Fame among people

It's likely that AOC and Ro Khanna, for example, only pretended to be against war to get anti war voters to vote for them.
It's probably the case that Biden didn't care about the climate crisis and never intended to update the infrastructure to go carbon neutral.

I think you have it backwards: in most cases it is their narcissistic tendencies that drive them to seeking fame. The rare ones thrust into public fame either become reclusive or surround themselves with "possies" to protect themselves from the paparazzi and general public, becoming isolated from the real world.

They never had the devotion to begin with. They lied to get into power because only insecure narcissists seek power. Look at any position of authority over people. It literally attracts the worst of humanity and there is only limited vetting on those positions.