Valor = People who dislike the faggy nu-male pretentious intro from Mystic and the boring egg-obsessed intro from Instinct, normies who like fire types, competitive players that want to do gyms and mainly battle Pokemon, people who saw how much hate Valor gets and thought the other teams were overly negative, cringy dorks that want to dominate people in a video game and tsuyoku naritai at PoGo.
Mystic = People who dislike the cringy red pill "I'm going to dominate this video game" Valor intro and the boring egg-obsessed intro from Instinct, normies who like ice/water types or pick whichever option is in the middle and therefore "most important", casual players that desire strength in numbers, gullible dorks that fell for the team bashing and believed Valor were dumb and toxic, pretentious nu-male faggots that want to demonstrate their superiority by choosing the most "intellectual" team and show how much they love science and rationality so they can virtue signal for ugly aspie girls.
Instinct = Nobody cares about researching eggs and the intuition of dumb animals, at most players would pick Instinct for the intro because it's the only one that doesn't try to inflate the game into something intellectual or something you can gain power and dominate people in, they'd rather go for the neutral and boring intro than the ones that might be construed as cringy, players that want to level up the quickest by having as many gyms available for them to attack as possible, contrarian hipster nu-males that choose whichever is the least popular, Pokemon dorks that think choosing a team with the bird that has the most metagame viability will make them look cool, normies who like electric types or the idea of lightning itself, "chill" stoner rejects that don't want to be part of the hostility between Mystic/Valor.
And reasons that apply to all: favorite color, because their friends chose it, at random, because they liked the bird the most, or because they liked the team leader.