Could they be leaving it later to stop people buying to sell on?
They would have less time to sell it?
They've got nothing to lose by making it later, really.
People return their tickets all the same and they know they'll sell as soon as they're put on sale.
I reckon they will have loads of people phoning up/complaining when the late availability is on, saying they can't get a ticket. So from their point of view, why not do that for a week rather than 2 or 3?