It's the fact that iMessage is built in... no need to go to the App Store to install any 3rd party app and then you come into issues where one friend has WhatsApp, while another friend don't but have Messenger.
Whereas all iPhone users have iMessage and there's no need to jungle multiple 3rd party apps... but this really all started with how carriers setup limited text messages plans and Apple swooped in to take advantage of it.
With the help of BlueBubbles or OpenBubbles makes switching to Android possible when iMessage is the main obstacle, folks can make the move if they actually want to.