Don't assume anything. Why would you assume that it would have been 6pm? Why 6pm? Is it because hotel bookings hold it till 6pm? Airline reservations systems operate on a different scale and when they say , say to issue your ticket within 72 hours, it means exactly that. 72 hours from time of booking. Not 72 hours plus 3 hours.

You left it too late to issue your ticket and well, sorry, its just bad luck that your booking got cancelled. And there really is nothing the airline supervisors can do to get your seat back. You may put yourself on waitlist and see if the booking classes open up again. They usually will but you'd be risking it at 48 hours prior to departure.

Basically know that your booking is not unique. No airline will guarantee the fare unless ticketed. So you should have just issued your ticket on the 30th!

If you are a CX elite member of their Marco Polo Club ie Gold/Diamond, your economy seat will be guaranteed even if its sold out. Sorry if that doesn't help you now, but you might want to consider being a Gold MPC member for next time!