Part of the reason for this is due to the direction of Steven Spielberg. The Color Purple was his first real foray into straight drama, and it gave him an opportunity to truly grow as a director. His signature moves as a director are there: one or more of the characters looking at the sky at some celestial orb, camera-shots that pull into tight close-ups, long training shots that make up the majority of a scene. But, here he finally employs them to tell a mature story dealing with emotional truth.
Although The Color Purple is one of the biggest all-time Academy Award losers (it lost all nine of its nominations, and Spielberg wasn't even recognized with a nom), it made Hollywood take notice of Spielberg as more than a genre director of very stylish popcorn flicks.