Some people want to communicate over Line, others Telegram, discord, a few on WhatsApp or Facebook messenger, and myself on Signal. I’ve gone down the path of installing now defunct apps like Allo before at friend’s urging, but I am too grumpy now to manage a mapping of people to preferred apps. Similarly, convincing all my friends, present and future, to use my platform of choice is a losing battle and managing threads between them invariably must decay to the lowest common denominator.
I think a better question would be why and how most other places moved away successfully from SMS despite this problem; and I think it’s because unlimited texting plans did not catch on with carriers there, so they had very strong economic reasons to prefer data messaging instead. Whatever app hit that market first became the new default.
In case it's not clear to anyone outside the US who reads this: unlimited SMS/MMS is available from virtually every carrier in the US including the budget MVNOs, and that's been the case for many years. But the "unlimited" data plans are expensive enough that there's still a significant market for cheaper plans with limits of a few GB per month—limits that SMS and MMS don't contribute to.
As a result, there are some people I can only chat with on Facebook Messenger, some people I can only chat with on Telegram, some people I can only chat with on WhatsApp, some people I can only chat with on Snapchat, some people I can only chat with on Signal, and some people I can only chat with on Discord.
I'm not sure that's so much better than a single standard which everyone on every device on any software platform can integrate with.
SMS just works without all that kerfluffle. And doesn't involve me with yet another service provider, as a bonus.