Actually Ryo and his backstory are a huge plot point and focus. It's just that like the CD Dramas, the sources explaining how and why Ryo is in the Tamers world never left Japan.
Ryo and his backstory in the Adventure World revolve around Millenniummon. Ryo is in fact first called to the Digital World when our resident Y2K bug (and the Dark God of Time) resurrects all of the Digidestined's fallen enemies and uses them to pwn the group. Agumon, left to die after the baddies abscond with all the other children and partner Digimon cries out and Ryo answers. One RPG later and Ryo saves the world--and Tai.
However, the EVIL Millenniummon has even planned for his defeat and hid a Virus/Virus Digimon (Depending on whether you're talking dubs or subs) that set off the events of Our War Game. (The second part of the Digimon Movie for Americans) Ryo is seen briefly in the mountains encouraging the Digidestined.
BUT even THIS was in Millenniummon's plan and caused a second, much weaker Dioboromon to spawn. Ryo is called back to kill it with the help of Veemon--yes, the same one that would be Davis's Partner in 02. But killing the Dioboromon revives Millenniummon, and the Digimon's resurrection causes the Digital World to be rent in two--also turning Tai and Agumon into statues. Then Tag Tamers happens. This is when Millenniummon's plot is derailed somewhat when Ken pushes Ryo out of the way and takes the Dark Spore himself.
Then while Ryo is trying to help Ken recover we have to deal with Anode/Cathode Tamers, where guess what... come on guess... Fine I'll tell you... Three of the Sovereign turn evil! Psych! That's just the explanation they give to Ryo as to why they needed a tournament. Actually, Millenniummon is back, again! Shocking, I know! At the end of the game, the island where Ryo and Millenniummon where fighting blows up and they are never seen in the Adventure World again.
Neither died however, Ryo was thrown into the Digital World's ancient past while Millenniummon was flung far into the future. But with Millenniummon's dark time powers and no Ryo to stop him he collects all the possible future timelines and rules over that apocalyptic dystopian future all the while attacking the distant past while also searching for Ryo. Ryo sets out to stop this with a Monodramon given to him by ENIAC, the first computer and the god of the Ancient Digital World. Because of Millenniummon's attempts ENIAC is able to influence any possible future that is connected to his Digital World. During Brave Tamer ENIAC sees Mllenniummon trying destroy the major players in the resistance in his Future--there are fifteen, the Digidestined from the Adventure World and the three main Tamers. Because he went after Takato first, Ryo befriends the boy who lends him Guilmon's aid.
At the end of Brave Tamer, Ryo and Milleniummon have a final confrontation where Milleniummon drops the bombshell he's been holding on to since Tag Tamers: HE, Milleninummon, is Ryo's TRUE PARTNER. THIS is why ONLY Ryo could DEFEAT him, but also why he kept COMING BACK! While Ryo is still alive Millenniummon will always RETURN! Monodramon calls BS and the Rookie and the Mega get into an argument about who is Ryo's partner while the kid is kind of catatonic. Then Monodramon initiates a Jogress, (which Millenniummon allows) and they both become one Digiegg. Ryo returns to ENIAC's timeline with the egg and is given a choice about if he wants to go back to the Adventure world or not. He chooses not, so he is sent on a one-way trip to the Tamers World. The egg, by the way, hatches into his partner in Tamers, Cyberdramon.