I don't understand why this was necessary given Swarm, Mesos, Kubernetes, AWS... Can someone elaborate?
I hear similar questions about Cloud Foundry: "why did you write Diego instead of using Kubernetes?" -- because Kubernetes didn't exist. "Why BOSH instead of Terraform?" -- Terraform didn't exist (and has different opinions, anyhow). "Why Garden instead of Docker?" -- Docker just didn't exist. And so on.
I call it NIYS: Not Invented Yet Syndrome.
Disclosure: I work for Pivotal, which donates the majority of engineering on Cloud Foundry.
AWS is IaaS. AWS does offer some 'PaaS' experience through the offerin go ElasticBeanstalk and ECS. But neither came close to a true PaaS.