It seems as though iMessage was omitted because unlike the messengers in the report, iMessage cannot be installed on any other platform but Apple’s own. But iMessage has a huge user base. I know a lot of people who uses iMessage as their primary messenger, they just use what’s app to communicate with people overseas, group chats that involves people who use android phones or to send large files. I myself fit into that category, if it wasn’t for those reasons I’d never use WhatsApp.
Add me to that list as well. If/when I use What's App it usually comes at someone else's behest. And TBH, I don't see the allure!?
iMessage is just so comprehensive, I've never had a need for anything else.