For me, ECS having less to learn was its main appeal. You get integration with AWS load balancers giving zero down time deployment and its API to automate is very straight forward. I set it up 2 years ago and have barely touched it. I evaluated K8s at the same time and after a full day was left completely confused how to do the same thing even if I spent a full werk.

ECS has definitely got some oddities mainly in the task definition spec which is most a 1:1 with docket commands but with their own AWS stuff mixed in. Part from that the simplicity vs K8s was its biggest draw card. Lot has happened with K8s in two years I’d imagine, so same choice today might be a different story.