>>35154156Because he actually has progression and is relatable. Starts out as your friend and overcomes health issues to better himself and become your rival.
Meanwhile the rest are just rival in name only as they're usually no more rival like than any of the friend characters with the exception of Silver and Gladion.
Silver has progression but is not relatable because most people don't just go around stealing shit and being a dick intentionally and you certainly don't become friends with them like you kind of do with Silver in the end.
Gladion meanwhile is just emo personified. no real development and cliched in what little lore we have.
