Why SMS is purpose built for IoT

With IoT cellular connections predicted to surge from 760 million to 3.1 billion by 2025 why is SMS, a technology first rolled out 30 years ago so well placed to benefit from this explosive growth? Here we look at:

What are the key technical reasons for the suitability of SMS to IoT?
What is key about the networks themselves?
Why the eco system around SMS is alive and well.