Why were these locked? Speculation suggests that Polyphony decided to move these assets to GT7 as "exclusive" content to drive upgrades to the PS5. For GT Sport players seeing the datamine, v1.68 was a painful reminder that the game is finished.
Below is a blog post draft covering the details of this specific update and its context within the game's history. Gran Turismo Sport Update 1.68: Tuning for the Future Gran Turismo Sport Update 1.68