The one-and-done era started back in 2006 when the NBA passed a new rule stating that draft eligible players had to be at least one year removed from high school in order to participate in the NBA draft.
This rule was enacted to slow down the mass flow of high school basketball players that had been flowing into the NBA over the past few seasons.
Some of those were great like LeBron James and Dwight Howard. More were not.
So since it was obvious that NBA GMs couldn’t pass on potential talent, to help the NBA, and to a much smaller degree help the player, they created the one-and-done rule.