Bone survives better than soft tissue.

Stone tools survive better than bone.

Most traces of human activity at that age have simply vanished.

Footprints are, in some senses, the opposite of what you would expect to survive.

They are impressions in soft material — by definition, material that was not hard when the impression was made.

They require a specific sequence of events to persist: the material must be soft enough to take an impression, then must dry and harden without being disturbed, then must be buried quickly enough to avoid weathering, then must remain undisturbed underground long enough for the sediment to lithify or at least become stable, and then must be exposed in a controlled enough manner that the impressions survive the re-exposure process.