TL;DR: Apple dominates the US smartphone market, but EU regulations may offer Android a chance for resurgence by enforcing messaging interoperability and standardizing hardware features.
100% of iPhone users have iMessage. And they can use the same app to talk to 100% of their contacts.
Fragmentation is a huge problem for everything else. What percentage of Android users have any particular one of those apps you listed?
I only grudgingly install WhatsApp when I travel to Europe. Discordv and Slack are not really competitors in this space (though I'm sure there's a small subset of users who use them that way). I have Signal and Telegram and yet I still use SMS with most of my contacts because that's the only one that is guaranteed to work.
I've tried getting my friends onto Signal, with some amount of success. But many have eventually stopped using it because I was the only one they used it with. A couple of my iPhone-using family members reported that they stopped getting notifications from Signal because they used it so infrequently that iOS basically disabled it (I guess it does that after a month of disuse but I'm not sure exactly).