A lack of funding which resulted in a lack of motivation and interest. We tried so hard to get to the moon so fast because in our childish minds we had to outdo Russia. That was the sole reason we tried so hard to get there. Because we had to beat Russia.
Well, then we got there, and everyone lost interest. Sadly, hardly anyone even watched the later moon landings, even though they had better cameras and better footage. But no one cared anymore, we had beaten Russia, we had made it to the moon, so people just stopped paying attention.
So finally, due to the facts that no one really cared anymore, that we had no one to beat (no one to challenge us), and that we felt we had already accomplished our goal the projects were cut, the funding started to slow, and the space program disappeared for a while. After a while, there was some interest in the space shuttle, but that faded. Then space stations, but that faded. Then mars rovers, that faded. Every successful mission after the moon landings was nothing more than interesting little novelties to people, things they forgot about a week later. All people ever cared about were the disaster missions. We focused more on the mistakes than the major accomplishments we had made.
And this isn't just an opinion. If you want some proof as to the fact that we only did it to beat another country, just look at the space program today. It was stagnant, doing some amazing but rather limited projects, for a long time. And then china announced its plan to go to the moon. Like the snap of a finger, we all of the sudden had a moon program again and poured $100 billion into it. And by the way, that is how much the entire space station cost, and we have yet to even go into space regarding the new moon program.
And no, our chances aren't over. They never are. There is just a lack of motivation. When we have a reason to do it, we do it. But we haven't really had any motivation to do anything. Still, we have gone to mars and done other things, even without motivation, so keep that in mind. But still, we haven't had any gigantic programs until recently, and the recent one is nothing more than a repeat of what we have already done. When the motivation and opportunity present themselves sometime in the future, we will make it happen, that I am sure of.
Plus, you have to keep in mind that we don't really have any reason to work on developing space travel technology or long-term space travel. We have no reason to simply fly around in space; we only develop space travel in order to get somewhere, so it has never been about the actual travel in space, but more about the getting to and back from places. Considering this fact, space travel isn't really important to us at all, just the getting there and the getting back. And think about it: we have computers and robots that can go to those far away, difficult places, so we simply don't need to develop that technology. It's much cheaper, easier, and safer to send a robot to another planet.
If the actual travel itself isn't really that important to us and the getting there is, then we have no reason to focus on developing space travel technologies, or really any reason to send people for that matter. All these projects depend on state funding, so consider yourself a senator who is going to approve NASA funding for a moment. If the scientists come in and say they want $150 billion to develop space travel tech and when you ask why they say "just because......so we can make spaceships..." You probably aren't going to approve that funding, right? You have no reason to build spaceships, and there is no reason for you to when there are cheaper, easier, safer ways to go places. You would probably rather spend that money on more important, more immediate concerns, like health care, poverty, the national deficit, preventing another depression, etc.
Until there is some immediate need for space ships, we won't be wasting money on building them when we could use the money for more important things, like feeding children and creating jobs.