Whatsapp is universally available as far as I know. If people choose not to use a universally available, free and secure alternative, it would seem that universality is not the main driving factor.
Network effects are a thing. Mere availability of WhatsApp (or Signal, or whatever) doesn't do you any good if all the people you actually need to talk to don't have it installed.
Yes but you can’t fault Apple for people freely choosing to use one app over another, especially when they do nothing at all to discourage whatsapp and the other third party alternatives.