Comment by MiddleEndian

When texting took off, it was the easiest (only) way to send instant text based messages between friends wherever you were, even if the phone system is now heavily used by spammers and there are better options.

When Facebook took off, every Myspace page was so full of garbage that they barely loaded on most people's computers, and Facebook was slick and shiny and easy. The real name policy made it super easy to connect with people you met IRL. Even if it's now confusingly slow and FB Messenger can't display your recent chats in the correct order for some reason, it was the easiest most obvious option at the time.

I don't really understand why people use Twitter (at its best it just seems like a worse version of RSS), but the site presumably loaded quickly at some point and was easy to use, even if it's presumably worse now.

And so on. They persist through momentum.

Some things continue to persist, some things get beat out and die. But if you start off more confusing than your alternatives, at least compared to when they started, you won't get picked up in the first place.