While players were free to enter the draft out of high school in the '90's, the NBA welcomed fewer teenagers than it does today. In fact, there were more one-and-done "student athletes" taken in the 2012 NBA Draft (10) than there were draftees out of high school in the entire '90s decade (nine).
It might not be fair for the NBA to require two or three years of college ball, but the impact that it would have for the game (both college and pro) is undeniable. College fans wouldn't have to endure never-ending roster turnover, while NBA fans deal with fewer undeveloped Josh Selbys, Shawne Williams', and Daequan Cooks. In the '90s, players created rivalries, developed more complete games, and became more mature human beings while in college. That's not a shot at today's youth; it's a tribute to having structure at 19 years old rather than a signing bonus.