And there are a lot of companies which do not become the next Unicorn and need an easy way to manage their container workloads.
SetOps currently uses ECS since it comes with no additional overhead costs for the management plane/API and does the container management job well enough. However this is not a definite decision and ECS could be replaced in the future. The main point is that there is a simple abstraction for users managing the workloads and that the "backend" is interchangeable.