IMO he stopped it because Russia wasn't meaningfully threatening to field a mission. The end was always artificial and if the USSR had beat the US to the Moon maybe we would have gotten a race to the first "permanent" base on the Moon or spacestation or even a rush to Mars. That's the point of the "did the US really win the Space Race?" question, it's about how the finish line was essentially always the next thing the USSR hadn't done yet.