K8S has a credible local development and testing story, ECS and ASGs do not. The fact that there's a generic interface for load-balancer like things, and then you can have a different implementation on your laptop, in the datacenter, and in AWS, and everything ports, is huge.

Also, you can bundle your load balancer config and application config together. No written description of the load balancer config + an RPM file to a disinterested different team.