Things burn out.
When most arguments have been made and counter-argued, then things tend to die out.
Existentialism in general is nowhere near as big as it was in the first half of the twentieth century (when it can be said to have occurred as a "movement.") I am not sure entirely about the reasons for this, but in particular I think the popularity of logical positivism (for example, Russell, Ayer, Carnap, Wittgenstein) away from the questions that the existentialism of, say, Sartre dealt with. Analytic philosophy became and is still generally predominant in American universities. In continental Europe, there was a shift in the 60s and 70s towards critical analysis of culture and history. Man emerged as a being in the midst of a culture, and so the study of culture became very important. It is in this vein that you get philosophical versions of Feminism and Neo-Marxism, which suggest that the entire tradition of western philosophy is an attempt to maintain a male or bourgeois ideology. Other notable cultural/critical theories were, of course, structuralism, post-structuralism, post-modernism, deconstructionism, etc.
In short--Existentialism as a movement or a philosophy that anyone really accepted as a doctrine is not really that big anymore, though it had an enormous influence. The history of philosophy in the second half of the twentieth century is pretty complicated, since many of the "schools" that developed defined themselves as having no doctrine (post-modernism, in particular.)
Buescher, an excellent post.
