There are many Twilio competitors and there have been for a while now. However, Twilio has been really resilient in terms of keeping their high growth year over year. What is the reason for that?
Reliability in the telecom space is a difficult problem, it's a large ecosystem of providers, aggregators, regulators, and lots and lots of hidden complexity. Then there's actually running the service, keeping the REST API up 24/7/365, making sure the TwiML processors are processing, keeping everything humming along smoothly for billions of requests a week.
When I was there, we would see "Twilio Killers" launch once or twice a year, normally competing around price. Their launches would almost always have the same 53 countries that they could deliver messages to, which we knew from operating in the space meant that they had just white-labeled Bandwidth's offering. They would be relying on a single aggregator and have the same reliability, resiliency, and reach as that one network.
There's a lot of reasons that people choose Twilio, network effects can't be denied, a lot of effort was expended on keeping it top of mind in developers heads. But that only scratches the surface.
We focused a ton on being reliable, being easy-to-use, and on backwards compatibility. There's a lot of unmaintained WordPress plugins that were written 9 years ago, abandoned 8 years ago, and still work fine because API compatibility was a top priority. There's something to be said about being able to just build an integration and then mostly ignore it and it just hums along sending SMS out reliably. A developer spending a day debugging a less reliable service can obliterate the savings in price, sometimes, depending on volume and use case.
Ease-of-use and network effects shouldn't be discounted. Most mainstream languages have a reasonably good library, Twilio officially maintained 6 of them while I was there. Our Developer Education team spent a ton of time and efforts creating tutorials, quickstarts, blog posts, and presentations. The Developer Evangelism team could be found at nearly every Hackathon, Conference, and Meet Up, spreading the word about Twilio, with well-polished live coding demos that would spark a lot of "Wow moments."
Even knowing exactly how every part of it worked, there's something magical about a presenter having everyone in the room text a number, then having a 3 line ruby program hit the API, pull down all the messages, and display them in the terminal. Then with two or three more lines of code, text everyone that texted in a message. That kind of demo takes maybe 10 minutes to do, but people would practice and polish it until it was seamless. You could look around the rooms at some of the events and see the wheels turning in people's heads as they started thinking about how they could integrate this functionality into their product.
First-mover, network-effects, reliable, resilient, easy-to-use, and in the beginning there were a number of highly promoted price drops to keep the momentum up.