Unfortunately, none of those services or projects existed when we first started building Atlantis at Ooyala. I haven't been at Ooyala for a while now, so not sure what their plans are for this project now that open source tooling with wider support exists.
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.