Many people would say that its because it was too late, but if you want to look at it like that, why didn't this apply to NASA when the Russians beat America in putting the first satellite, animal, man, woman and space station into orbit? Russia would not have thrown in the towel just because America had beaten them at one single thing in Space!
Not one thing that appears on the surface of the Moon had to be placed by Man. Be it mirrors to reflect lasers from here on Earth to calculate distances or seismology equipment. All could have been placed there by robotic machines. It wouldn't necessarily need a human to place them there.
Graham Birdsall (the late Editor of UFO Magazine UK) commented that during the very first Pacific UFO Conference in Hawaii in September 1999, Astronaut Brian O' Leary who worked alongside the likes of Neil Armstrong and Buzz "Sucker Punch" Aldrin on the Apollo 11 mission during 1967-68, commented ' If some of the films were spoiled, it is remotely possible that they (NASA) may have shot some scenes in a studio environment to avoid embarrassment!'
During Project Apollo, six highly complex manned craft landed on the Moon, took off and returned to Earth using a relatively low level of technology. An 86% success rate. Since Apollo, 25 simple, unmanned craft with increasingly higher levels of technology have attempted to fulfil their missions to Mars. Only seven succeeded.
Russia have and continue to have unmanned craft launch into space. Their project worked more towards first sending an unmanned lander to the Moon surface to collect samples, take pictures and measure radiation levels. Why did NASA not even send an unmanned craft to the Moon surface before risking human lives?