In a nutshell: because you can't do everything at once, and although the Moon is indeed interesting the rest of the Solar System beckons. From the 70s to the about 2000(-ish), the US space effort was focused on the rest of the Solar System (see for instance, this chart of the distribution of JPLs missions by year; and this one too, non dated but a bit more exhaustive; and cool...).

And there are a number of subsidiary considerations to take into account whose importance vary depending on what you mean by "we" and "been back". Are you only considering manned missions? Only american missions? Do landings by robotic and/or stationnary probes matter? Do orbiting satellites such as the Lunar Reconnaissance Orbiter (2009) count?

For instance, one highly visible element of the US space program was manned exploration. While this was an interesting driver of technological innovation, it has rapidly become obvious within the exploration community that the constraints imposed by manned flight were an unnecessary burden on the capabilities deployed versus the costs. For instance, it currently costs in the order of $30 000 to get one Kg of stuff in LEO (t'was more in the 60s and 70s); when about 80% of that payload is life support, it really puts a crimp in what analytic capabilities you can get to your target. Especially considering the advances in computers, telecommunications and robotics in the last 30 years. You need only look at the martian rover programs to see concrete evidence to that effect.

There have been a number of lunar missions since though (although none was manned), and they have received less press than the Apollo program; these were notably sent by Japan, the ESA, China and India.