Hip hop had a few mainstream breakouts in the 80's. And of course in 1990, MC Hammer and Vanilla Ice opened doors to those white suburbs to hip hop despite them doing a watered down version.
The TRUE turn was the end of 1992 with THE CHRONIC. It was the 1st hardcore rap album that had crossover appeal.
This introduced the West Coast as a WHOLE to dominate the mainstream and radio until READY TO DIE, by Biggie's own account was the East coast version of The Chronic (or he wanted it to be) which pushed the East Coast to join the mainstream affair.
To put it shortly, the transformation of mostly underground to mainstream was between 1992-1994.