Because he is the digimon that started it all..the pioneer of all digimon.

A Digimemory is something like a memory stick that can be used to summon “legendary digimon” to help in battle.

The Digimon represented by the DigiMemories were a chivalric order that once protected the Digital World but failed to keep it from being torn apart, and as a result were transformed into DigiMemories and scattered throughout the Digital and Real Worlds. (source)

As you can see, the legendary will only come out to use their skill (fixed move) once and disappear, and the Digimemory can only be used once in each zone. Agumon’s attack is nothing to brag about, but it made me remember his awesomeness in Digimon Adventure, the first season. But of course, Patamon is still the one and only best. I hope Patamon gets a Digimemory too!