Not always the case, look at the soviet union. Collapsed because the soviet leadership lost faith in what they were doing.
#Venezuela is a great reminder:
You vote your way into communism, but you have to shoot your way out.
Discussion
You left out the part where they murdered a quarter of the population then utterly bankrupted the country by running an economy based on political favors for 3 generations. But yeah for brevity's sake I guess "lost faith" sums it up.