Not saying this is the reason but would they know which sponsors tickets, etc. definitely won't be taken and will they always be for the same seats at every match?
I think that some arn't available until very late due to being held back for sponsors, guests of players / officials, and for a possible larger away section if needed, etc. and therefore can't be released for sale until the club know for sure that they arn't wanted and they have been returned to...