why has not Nasa gone back to our Moon since the first landing on the moon
We didn't only visit the Moon once! Neil Armstrong, Buzz Aldrin and Michael Collins were the first manned mission to land on the Moon with the famous Apollo 11 landing, but 5 later Apollo missions also landed safely on the Moon and did various exciting things. This Wikipedia article gives a brief list of all the manned and unmanned missions that have investigated the Moon, and this NASA article gives a nice history of the Apollo program with all the manned missions. (So far the US are the only country to land astronauts on the Moon.)
As for why there haven't been more visits since Apollo, there are a couple of points:
1) We have continued investigating the Moon, through remote observation, close encounters and landings, just not with humans on board. The future of human space flight is quite a debated question, since humans are very fragile, have lots of requirements in terms of food, space, health etc. and are not really very efficient at many tasks. Automated investigation of space is by far cheaper and easier. The main advantages humans have are intuition and adaptability. We can change plans, make snap decisions and respond much quicker than a robot controlled by people far away on Earth.
2) Turn the question around: why would we go back to the Moon? Why spend money and time investigating this rather boring lump of cold rock when we could go to places we have absolutely no data on, or very shaky theories, or places that have environments hugely different to our own? If we want to test scientific theories, we often want extreme environments, not just something a little bit different to home. And if, as your comment says, we are looking for other life or future homes, these things are very rare so we will have to look a lot further away than the Moon to find something that interesting!
Nasa has not gone back to the moon for a few reasons. Nasa was no longer being funded for the apollo missions, but also don't want to risk the lives of astronauts. The political aim of the apollo missions was to make sure every astronaut returned alive, and the scientific aim was to learn as much as possible about the moon - they would love to have someone up there for weeks!
Something NASA has been working on is a virtual simulation environment of the moon. They believe this can help save the risks of any lunar accidents and allow them to study the moon safely from earth.
I did a few google searches to find this answer, so information might not be exact.