Another possibility is that every streaming service wants "Pokémon" and parents don't care which season.

So each service buys a single season to tick that box.

This. Pokemon likely has an a la carte menu and platforms can choose which seasons they want.

Streaming services have a limited budget for kids content so they can’t catch em all without sacrificing other kids content. They need pokemon they need action they meed rainbows and princesses and they needs stuff for babies and toddlers.