Because of ubiquity - there are too many third party apps, and I don’t know which ones to reach you on. But sms is preinstalled on your phone. If your phone is an iPhone, iMessage works better than many of the other third party apps anyways and if it’s not it degrades to sms and you still get the message.

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.