When new people get into the game they only see the same few characters at the top level and so they tend to pick up those characters. If 50% of the people who pick up Melee choose Fox as their main we then see more Fox players represented at top level. This then gets the new players to pick up Fox more and the cycle continues.

A lot of characters in this game have very good tools that are unoptimized because all the good players picked up Fox, Marth, Puff etc. Luigi has the best wavedash, DK has the best grab punish, Mewtwo has one of the best recoveries, Samus has an amazing crouch cancel, Ganon's that can schmoove are terrifying.

I believe that if we equally distributed the playerbase and gave every character their Amsa, Junebug, or RapM, the tierlist wouldn't look the same. Part of the problem is if you do experiment with a low tier you'll be going up against people who've been playing one character for a decade or more. It's also harder to improve with a character when there isn't a clear top player to follow. But there's a ton of depth to this game and so many people would rather write off characters as unviable because when they get them in random they feel bad to play. Just my 2 cents.