And what would be the point of starting selling them earlier?!?
Don't you think the distinct smell of relegation would have put any casual fans off being tempted?! The loyal fans will obviously buy one regardless.
If you put them on sale early you get an head start. And have more time to sell them , which gives people more time to think about wether they want one or not.
Look at burnley they have already sold 8,000 s/t.