Sure, there's some wonderful chat clients out there, but the average person wonders why they need any more when 99% of their friends, family members, frenemies, estranged highschool classmates and so on are already easily reachable on Messenger.
Even the most wonderfully accessible and user friendly app is still another app that I need to take the time to understand. It's another app for which I need to endure the endless parade of 2.0 rewrites and UI overhauls. It's another app I need to figure out where I want to place on my home screen.