They actually never forbade it, but the bishops were instructed to replace it with the new rite, which supposedly would not be very different from the old one. There was a brief period in the late 60's when we had a rite that was basically a translation into the vernacular of the Tridentine Rite low mass, with a few tweaks. And then came 1970, when the Novus Ordo mass (along with the calendar changes, etc.) was suddenly dropped on us. I was at my parish church the first day it was celebrated - a weekday - and most of us left the church in tears because of the flat and ugly thing they had presented us with. And this was even before "clown masses" and the other abuses that appeared almost immediately.
The reason they wouldn't permit a choice is that the modernists knew which mass people would choose - and it wasn't going to be the one they wanted to impose.