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.
That's also true of "managed" Kubernetes platforms like Google Kubernetes Engine and Amazon EKS, the difference being no (or at least less) vendor lock-in.
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.