"At the beginning of this voyage, Chekov has expected it to be boring, but short and easy. How difficult could it be to find a planet with no life? Now, several months later, he felt as if he were trapped in a journey that was boring, unending, and impossible. Lifeless planets abounded, but lifeless worlds of the right size, orbiting the proper sort of star, within the star's biosphere, in a star system otherwise uninhabited; such planets were not so easy to discover. They had inspected fifteen promisingly barren worlds, but each in its turn had somehow violated the strict parameters of the experimental conditions."
From the novelization, which ain't canon of course, but serves as an answer well enough.
For my part, I'd say that the year 1996 went by without eugenics wars bring waged is hard to explain