Why do so many Alolan forms of Pokemon gain a secondary Dark typing? I would think a bright and sunny place like Alola would do the opposite of giving Pokemon an affinity for the dark.

Not necessarily, anonymous. In some cases, primarily when it comes to the rattata and grimer lines, these pokémon had to hide beneath buildings until nightfall to avoid extermination by humans or hunting by other pokémon. Thus, they adapted to the darkness of Alolan nights, rather than the brightness of Alolan days.

The meowth line, meanwhile … no one’s quite sure how selective breeding managed to give it an affinity for the dark type, to be honest. But given how adept standard meowth are at conning their owners into giving them more treats or affection, it’s perhaps not surprising…