I think its more one of those, use what their friends use, kinda thing
They’re not going to be happy using an app just to communicate with one person
Speaking for someone with a huge majority of my friends living overseas, iMessage is the one absolute must have for any OS. Sure you can get third party apps, but they're just that - third party - which is a total nuisance when you have to get everyone to download them. iMessage "just works".
iMessage tends to only be big in the states and some parts of the UK. Most other countries I know use the other services mentioned. if you have iMessage you receive messages faster, don't need to have a data plan or a text plan, sends higher rate photos and videos much faster and group chats are better and the main issue is texts between Android and iOS often get lost...Especially if you're in group chats. People with iMessage will get every text while it is not the same for a person with an Android in that group chat.
It's all well and good if all of your friends and family also use iPhones. If you have any contacts using Android, iMessage still can't be your one and only messaging solution anyway. That's why 3rd party apps like Facebook Messenger and What's App are incredibly popular, in my experience at least. They're cross-platform.