Timeline.

[Guilmon and a shotgun]

Fads come and go, give it a few years or so of no one caring and it wouldn't be a surprise if no one remembered them. And DMX that makes sense about them being in the east, but then what about the global tour arc where they traveled to the four corners of the planet? Seemed like Azulongmon was sealed away or something by the destiny stones, or whatever they were called, and as they were destroyed he was freed, so maybe the 3 others are also sealed by stones in their quadrants possibly to protect the stability of the digital world or something. All speculation but it makes sense.

New Evidence: During Episode 32 of Tamers (Shibumi Speaks), after Shibumi informs Takato, Henry and Terriermon of the Digimon Sovereign, Takato says "If they're like Azulongmon from the TV Show they must be huge!". Takato is aware of Digimon as a TV show and as we've agreed that Digimon 02 distinctly follows on from Adventure it's very plausable that Tamers is either a follow up or a closely related alternative universe.
As for the Sovereign, it could be that Aluzongmon found it difficult to be sole guardian for the Digital world, as it probably grew into an abundance of plains for vast varieties of digimon to thrive, and had the Digi-Gnomes create three other sovereign digimon to aid him.