I don't think it's a case of forgetting, but you're never going to be able to turn that car park back into platforms, at least not without creating another problem - parking! Given the cost of parking there, I imagine it's a bit more of a cash cow for LNER than the platforms would be.

Interesting about the GPO tunnel. I suspect there must be a reason why it hasn't been repurposed, given that it would be an ideal backup to the existing lifts at the very least.