I think it's probably because users paying to be able to use third party clients can't be sold as growth potential — it's a relatively static number, and they're paying for an entirely static experience. They don't want whatever gimmick Reddit is pushing at the moment (like NFT avatars), they want their simple chronological feed and that's it. The limit of worth for those users is the subscription they're paying, whereas if they're using the official app they're theoretically monetizable in N different ways.