Nicholas was barely 50 at the time of his death, and apart from smoking had lived a healthy life throughout his adulthood (moderate eating, relatively little alcohol and plenty of exercise). It may simply be that the smoking had yet to catch up with him. For comparison purposes, George VI had a lot of digestive trouble as a young man, which seems to have been largely resolved by an operation for ulcers in 1917. The smoking (and a generally poor constitution) caught up with him in his early 50s, from 1948 (first arteriosclerosis, then lung cancer). Romanov men weren't long-lived, so it's quite possible that Nicholas's health would have declined within a few years.
Ann