There aren’t many telemetry transfer protocols suitable for IoT besides MQTT (especially for resource constrained systems). It is incredibly simple to implement, has numerous libraries available in various programming languages, is supported by major cloud providers and is designed to operate over TCP (meaning you avoid DTLS).

We have shipped many products that use MQTT for telemetry transfer to various different vendor servers and it always just worked. It was never a pain-point in stark contrast to CoAP or some proprietary protocols.