Regarding to your quote MQTT is the "protocol of Internet of Things":
Yes, there is a great amount of developers using this protocol (see IoT Developer Survey 2018) but CoAP (it's HTTP adjusted for IoT, based on UDP) provides an alternative for HTTP in case you want to use a lightweight Request/Response functionality within your application.
MQTT on the other hand provides a built-in Publish/Subscribe logic, which makes it great for scaling (you can use more gateways for a greater amount of devices). There is also an UDP alternative (like CoAP to HTTP) that is called MQTT-SN (MQTT for Sensor Networks). This provides even a smaller overhead than CoAP, but doesn't make use of R/R.