With Monster Hunter, I think it's important to remember that characters aren't the only way franchises are celebrated and honored. To Sakurai, stages, assist trophies, and bosses are more on equal footing to playable characters than how fans primarily see it. For Monster Hunter, the most iconic aspect of the games is the monsters, so including a Rathalos boss as its representation makes sense. Just as Minecraft's most iconic element would be becoming a stage (though, that looks like that may not be the case). I think Sakurai looks at the highest selling franchises, fans desires, and upcoming promotional games, and tries to find the best way to include as many as possible, while maintaining their most iconic elements, playable characters being one of many ways of accomplishing that.