Many rates, not just at Hilton, require a credit card for the reservations to be held. Once you pass the cancelation time, some properties will authorize the charge to the room to determine if they will hold the room for you or not. It prevents you from using say a debit card to hold the reservation, deciding not to show up, and then just taking the money out of your account so they can not process a charge.

I've only had a few, very few, properties actually authorize the card at the cancelation time, one time the card didn't authorize (expiration issue like you had) and they gave my room to somebody else on a late arrivial.