The big selling point for k8s vs ECS is that it (theoretically) allows you to lift and shift your cluster to another cloud provider without too much pain because load balancers/storage/etc sit behind an abstraction layer. When you are AWS that's not really something you're even going to consider, so you may as well build with the underlying primitives because its absolutely guaranteed no one is ever going to ask you to migrate to Google Cloud.
ECS is 3 years older than EKS (2015 vs 2018) and a lot of AWS services launched (or planned to launch) during that time which is a very valid reason lots of customers picked ECS.
We've been running on ECS for 6 years now (basically within a few months of GA). When EKS launched we talked about maybe migrating over but ECS just feels more first class wrt cloudformation and stuff. It makes sense that internal teams rely on it.
Literally everything is a Lambda nowadays it seems, so it seems like it's just Lambda -> ECS -> -> -> raw EC2 unless you're building directly on bare metal.