Messaging network effects, if you're in an iMessage conversation and an Android user joins, suddenly the conversation becomes SMS only with photo and video sending being crippled.
Its the same as a social network, if all your friends use it, you almost have to in order to communicate with them.
This always comes up when non-Americans talk about US messaging systems. The simple fact is SMS was good enough (being pretty cheap or even free for many years) that people didn't care to switch. In contrast, many other countries had much more expensive SMS so free messaging systems were built and widely used.
Its the same as a social network, if all your friends use it, you almost have to in order to communicate with them.