In part, Moscow goes on with this because submarines are something it was good at in the cold war. The new Akula class, extremely difficult to detect and powerfully armed, was giving western admirals nightmares as that conflict ended. In part, Moscow goes on with it because the United States' approach to nuclear arms control is lackadaisical and compromised by the recent infatuation with national missile defence. Finally, of course, there is Vladimir Putin's desire to reform and strengthen Russia's military forces as a means of restoring its prestige, influence, and national pride. There is a dangerous irrationality here, but it is not just Russian.
Nuclear submarines are on the technological frontier.
The French probably have similar problems. Chinese nuclear submarines rarely put to sea. When they do, fingers are crossed. Other countries, some of them even less capable of controlling the technology, are beginning to hanker after nuclear submarines. It is probably true that the only navy that can run a fleet of nuclear-powered submarines efficiently and safely is that of the US, but even there accidents have happened.