When a company does not approve your request prior to their minimum lead time needed based on your event date, the request is automatically canceled for non-response.
Similarly, if you receive an offer and do not accept it in time for it to be delivered (based on the company's own internal lead time requirement), the offer is automatically canceled as well.
It is typical for companies who receive many more requests than they can approve to let requests they cannot immediately answer sit in their queues, just in case they are given the ability to approve them later. When this isn't the case, the auto-cancel happens.
In general, items that will be shipped or delivered typically have longer lead times, although some companies simply prefer to make decisions well in advance, and these timeframes may be communicated on their websites.