Existentialism, it has been said, was always going to fail because it requires people to take total responsibility for their actions. This is largely true. Many people are of the opinion that they have an essence that defines them, a non-physical substance that is them. It is the way that they have always been. It is who they are now. It is who they will be tomorrow.
You'll be able to figure out these people when they use expressions like, "But I'm not like that." or "I've always been this way, and I'm never going to change." or "I've learnt so much about myself." It's almost as though they have separated themselves into two parts. One part is the "I" that is making the comment. The other part is the "me" that is waiting to be discovered. But let me ask you, which is the "I" that has free will and makes choices? Is it the "I" or the "me"? Does the "I" take orders from the "me"? Where is the autonomy of that person?
Other manifestations of this belief are found in the idea that every life was created to complete some task; a telos. If this is the case, how do we determine what that task is? Typically the answer to this is something banal like "follow your heart." Unfortunately, there are a great many people who cause harm to others by following their hearts. Robert Mugabe sincerely believes that he is helping the Zimbabwean people. George W. Bush sincerely believes that he is helping the cause of freedom (whatever that means). Following your heart to find your cosmically appointed task is an act of self-indulgence, coloured with claims of divine right.
The alternative to this is the primacy of choice. That is to say, each person has the capacity for free thought, for spontaneous thought. Human consciousness is capable of creating ideas from nothing; ex nihilo. We are capable, in each and every situation, of choosing to do something that we have never done before, or something that is vastly unlike anything we have done before. A pacifist can choose to shoot another person in the head. A heterosexual man can choose to engage in homosexual activity. A man who gets into drunken fights can choose to lower his fists. Each and every example like this is possible through the primacy of choice.
And beyond that, the consequence of such choices perform two functions. They define the person who performs the act (not the other way around), and they become an irrevocable part of that person's history. The agent of a free will action cannot hide from that action. To do so is to deny that they have free will, the power of choice. Of course existentialism fell out of favour. It leaves us with no one else to blame for our mistakes. The only room it gives for us is the infinite potential of our next free will choice.