Presumably because of all the reasons they chose Kubernetes?
FTA:
* Kubernetes has a huge amount of traction in the DevOps landscape, with managed implementations from all the major cloud vendors and virtually endless training materials and complementary technologies.
* Kubernetes is open source, which was a major plus: it meant that we could avoid vendor lock-in and implement local development environments that mimic production.
* Kubernetes has a large feature set that fit well with our requirements, including our more exotic necessities like autoscaling based on custom metrics.
AFAIK none of those are true of AppEngine.
