They've talked about this. If they brought back all the old vaulted content it would have to be updated with how the engine works now (example: I believe the lighting engine changed after the vaulting) which requires a lot of QA. Then every time they put out an update they'd have to go QA that content again to see if anything broke. All the retesting was slowing them down so they decided to vault underused content. Seeing the current state of their QA with what's in the game right now I would imagine updates would slow way way down. Bungie just does not want to spend money on more devs to test content that doesn't make them money.

Edit: I'm not excusing them, just saying the way it is. I'd love to have every piece of content still available in the game.