>>365163331) Clefairy is far too feminine to be the mascot .It's pink and girly. Pikachu appeals to both boys and girls, having a gender-neutral look.
2) Clefairy is pink, and there were far more pink mascots at the time than yellow ones. Pikachu's yellow coloration made it stand out far more since the only mascots at the time were The Simpsons. (SpongeBob didn't exist yet.)
3) Pikachu's type was far more intuitive to understand. Clefairy is Normal-type, but nobody knows what a "Normal" type is supposed to be. Clefairy is pretty much a blank slate, but the anime was written with elemental powers in mind. Normal is the most basic and uneventful type of all. Pikachu is much easier to relate to; it shoots electricity from its cheeks and tail. This elemental association with Pokemon became a lot stronger in the anime when Ash caught the starters, Team Rocket used poison-type Pokemon, and Misty used water-types.
