I think it’s pretty obvious. Spin-off games will never be as popular as the main series no matter how good they are because they don’t present the same draw that originally captured the fans of those games.

Not sure if you are purely talking about pokemon but there are a good amount of examples of spin offs surpassing main line games.

Biggest one is probably Persona. Raymans Rabbids at this point have overtaken Rayman and become the mainline for that line of games. You also have stuff like the Mario spin offs like Kart or even in the case of pokemon you have pokemon GO being a giant cash cow and having a time period where it was practically the biggest game out there.