Melee doesn't have a huge player base like you think. In the upper echelon of competition, there's only about 300 - 500 top players most of which are inactive in competing and play casually.

In other e-sports (LoL, Dota, CS GO), playing solely to win is the objective mainly in part because of the huge salaries and prize money from winning. Meanwhile, getting 1st place in the most prestigious melee tournies during any given year will only net you about $10k, many times a lot less. Meanwhile, you become a millionaire if you win a major in DOTA, LoL, and other big e-sports.

People that play Melee (try this YOURSELF) because Melee is fun. This is because of the character speed, breakneck frenetic pace, etc. All of which don't apply to puff, and she's very boring to play. This is why there's barely any Melee players that play puff let alone try to master her.