>>723921118
A lot of Digimon probably aren't worth keeping around. For instance, many of the stuff from C'mon Digimon have probably been abandoned. Also, Izawa and Yabuno seem to own certain Digimon like UlforceV-dramon Future Mode, which means Bandai can't use it without their permission. There's also a handful of Digimon that plainly weren't designed to be used more than once, like Agumon Burst Mode.

A large chunk of unregistered Digimon are just random Digi-Xroses from Xros Wars. It's be pointless, redundant and a pain in the ass to catalogue all of them, so the majority have been abandoned.