Why we haven't been backto the moon?

Because we're no longer able to do it now.

The primary reason we haven't isn't so much capability but money. NASA's budget is currently about 0.5% of the federal budget. During the height of the Apollo program that figure was about 5%, with the vast majority being funneled directly to Apollo.

Throw enough money, national pride, curiosity and anti-Soviet sentiment at a problem and it will get taken care of in a hurry. It was the right time; a perfect storm, so to speak.

The goals, among other things, were making sure a human being could survive space travel and then, hopefully, to send a few to the moon. Those were accomplished (and, to be fair, the fruits of that labor continued to serve us well into the future).

The problem was that after these were accomplished people began to wonder why we needed to continue funding the same mission over and over again. An expensive mission. Adjusted for inflation, each Apollo landing (not mission, but landing) cost roughly $20B. To put that into perspective, that's more than NASA's entire budget this year alone.

A human standing on a surface other than the Earth is a truly amazing feat for not only our country but our species as a whole.

But, as amazing as it is, it's also extremely pricey and extremely dangerous. A lot of what we need to do in space can be accomplished at the ISS, and a lot of what we need to know about the moon can be accomplished with a probe. We need a more pragmatic purpose to justify the costs.