The reason why no one has walked on the Moon since 1972 is because of a decline in public interest after the initial Apollo missions as well as budget cuts and shifting political priorities.
Although the technology to return to the moon and walk on the moon still exists, the high cost of the technology and the lack of the sustained political public support has prevented further human missions of walking on the moon.
Only 12 humans have actually walked on the Moon and all of the 12 humans that have actually walked on the Moon were American astronauts that participated in the Apollo program.
No astronaut has ever walked on the Moon twice.
Each astronaut that has landed on the moon and walked on the moon only walked on the moon just once.
Several astronauts have traveled to the moon more than once, although only one of the trips to the moon, actually involved landing on and walking on the surface of the moon.
Jim Lovel flew to the moon twice, which was once on the Apollo 8 where he orbited but didn't land and once again on Apollo 13 which he orbited but also didn't land on the moon due to a malfunction.
Astronauts John Young and Gene Cernan also flew to the moon twice, but they only walked on the surface of hte moon during their second mission to the moon.
And the other 9 moonwalkers each only made one trip to the lunar surface.
Jim Lovell, David Scott, and Eugene Cernan (e: and John Young, oops) all went to the moon twice though none of them landed on it more than once.
Humans have walked on the moon a total of 6 times, with a total of 12 astronauts participating in the moon landings.
All of these missions were part of NASA's Apollo program between 1969 and 1972.
Human Moon landings require more resources than robotic landings, because humans require, oxygen, water, food and other amenities to remain alive.
So any country or agency which chooses to land people on the Moon will also have to accept a certain amount of risk and budgetary commitment.