Depends on the reasons indeed. Such as:
- Have the film cans all been sorted out? (Remember, there's a LOT more than Doctor Who
- Are the film prints the right ones, or are they in mislabeled cans?
- Are they in any kind of salvageable/viewable state? (Vinegar rot was mentioned.)
- If salvageable, how long/how much effort will it take to bring them to a DVD-worthy state?
He may be waiting until he's certain he has something to give them - he'd rather turn over full stories instead of piecemeal episodes if at all possible, and in a restored DVD-ready state.