I've wondered that before too. My guess is that it's far more expensive and dangerous than a plain old trip into space. That, combined with there being very little practical (read: economic) value to it other than as an achievement, made it a low priority once it had been done. Of course there are ideas for putting the moon to use now that are pretty practical, like using it to launch a mission to Mars, among other things, which is probably putting the Moon back on our radar.