I don't know if I'll ever have Jocun state this, but I did come up with a reason the Yeerks won in his dimension. During Book 8 when Ax tried to kill Visser three via snake venom, they did succeed in killing the guy. Visser One was put in charge, and eventually managed to capture or kill the Animorphs and take over Earth. V1 also found the morphing cube, and used morph capable controllers to infiltrate the Andalite Homeworld. By the time they figured out the Yeerks had an escafil device, it was too late.
If anyone was curious, there's the explanation. Since Jocun will try not to divulge anything that could change the timeline too much, he isn't likely to tell anyone.