Playing devils advocate here..
its because Leon's style is all about heart. He chose charizard out of style and a sense of power, and showmanship. He's a star, and a star has a charizard.
Yes, its pandering to the charizard love, but thats exactly what it is. its not concealing its intention at making the champion kanto-bait, Leon himself is a kind of charizard fag, and was probably KANTOOOO himself when seeing one for the first time.
imagining Leon without a charizard in his team, and instead a galar rep, wouldn't work, because his theme and quite frankly, Galar's theme is adulation and vocation, as told by the writers.
its why the gyms are huge, the crowds are roaring, the antagonists foible is his role and passion to his craft, and the starters are all based on entertainment.
Leon was always gonna have a charizard, because he is, and always will be seen, as the coolest pokemon, and the real star pokemon.
Think about it as a kid. How cool is it to see that the new champion's big bad is a charizard? you'd think that the new champion is cool as fuck.
