Reservations for popular national park campgrounds are frequently gone within minutes of release. The speed is a product of how the release is structured, not of unusual demand on that day.
Supply is essentially frozen
Most park campgrounds were built decades ago and have not expanded. Adding sites inside a park requires environmental review, new water and sanitation capacity, and land that is often unavailable.
Some capacity has been lost instead, as older loops close for hazard trees, failing water lines or floodplain problems, and reopen with fewer sites than before.
So the number of sites is roughly fixed while the population that wants them, and the ease of learning about them, has grown considerably.
A single release time concentrates everyone
Booking windows open at a set hour on a set date, which means every interested party in the country arrives at the same moment.
Demand that would otherwise spread across weeks is compressed into a few minutes, and the outcome is decided by connection speed and preparation.
Rolling windows, where a new date becomes available each day, produce the same effect daily rather than removing it.
Vehicle size filters the inventory further
Sites carry length limits set by the turning radius and pad size of the loop, and older campgrounds were laid out for much smaller vehicles.
A traveler with a large trailer is competing for a small subset of sites, which sell out faster than the campground as a whole.
Tent campers face the opposite situation and often find availability where recreational vehicle users find none.
Cancellations are the practical second chance
A meaningful share of reservations are released before the trip, since people book speculatively when the window opens and firm up plans later.
Those returned sites appear continuously rather than at a scheduled time, so repeated checking in the weeks before a trip works better than a single attempt.
Cancellation policy shapes this directly, because a system with a low penalty produces more speculative bookings and more returns.
First-come sites still exist
Many parks hold loops or entire campgrounds outside the reservation system, allocated on arrival. These favor early risers and flexible schedules over fast typing.
Surrounding national forest and state land often holds dispersed and developed camping that never fills the way in-park sites do, at the cost of a longer drive each morning.
Planning around those options is generally more productive than trying to win a release, because the release is a contest the system is not designed to let most people win.