SMS used to be really expensive and limited feature wise. Sending messages over IP ment that you did not have to care about how many messages you sent and you could send media with non potato resolution. Media over the phone network (MMS) was basically limited to small images.
Since the messaging apps were developed by third parties (and not your phone company) there was a market for several of them. The phone companies tried to counter with RCS, but being phone companies and not internet savvy pioneers, it was a slow process to get wide adoption.
And in addition to their terrible UX, they are also horrible software maintainers. This will definitely rot over time.