Timeline.

[Shadowknight]

Or it's possible that tamers was the very first chronologically and since they talked about actually creating the digimon and the digital world. Also in tamers, there were all four of the sovereigns, however in the other seasons, mainly 1 and 2, only azulongmon appeared, if the digital world was threatened that badly don't you think the other three would show up unless they couldn't?

Thats true, but how would you explain the strong presence of Digimon as a franchise in 03, but suddenly less abundant in Adventure? Surely Digimon fighting in downtown tokyo would have done SOMETHING to the sale of Digimon products and surely even the card slashing technique would still be used by Taichi and co.

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.