I wonder that as well. When I ask the question out loud, people usually answer "Well, everything looks so much worse now". I don't think that's the real reason -- Star Trek aired as the Vietnam War heated up, during an era of high-profile political assassinations, and it reached mass popularity in the 70s, which no one at the time regarded as a high point.

What's changed is that we no longer believe in the future, that the future could be better than the present.