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It has been made clear many times that Blossom is the smartest and toughest, hence why she is the leader.
She's also the only one who can match and beat Princess in her Powerpuff forms, both true and artificial.
Buttercup doesn't even have a special unique power like Blossom's Ice Breath and neither Aku, Dexter, or Jake Goldman are interested in her because she is weak.
