First place, I'd lose my deposit if I cancelled a room with the housing authority a few weeks before the convention if I booked at another place that becomes available.
Second plance, rooms are rarely available for the (large) conferences I'm talking about anyways.
Third, any "backup" that I could reserve and then cancel would probably be a mile or so away from the convention center (probably more like 3 miles away). Not an option.
Fourth - in New Orleans, a big hotel near the New Orleans convention center (Wyndam I think) is closed for *6 months* because of a termite problem. That has had a big effect on inventory.