It's the J part of JRPG that has, in the past, influenced Sakurai. Because he is from and lives in Japan he's given characters that are popular there (Jigglypuff, literally every FE character) and that are in Japan exclusive games (i.e. Marth and Roy in Melee and Lucas in Brawl) a bit more standing than Western characters. Only three playable characters in Smash (Diddy Kong, Dark Samus, and K. Rool) originated outside Japan. It's only starting with Smash 4 that he started paying attention to what the West wanted, and he appears to be going all out with it in Ultimate. That's what makes me think that characters with large, primarily Western fanbases (i.e. Isaac) have a significantly better shot at making it in then they have in the past.