Probably because you need to have an "investment of time" much bigger tryin to adapt your infrastructure/app to Google App Engine compared to using a simple Iaa

Probably because you need to have an "investment of time" much bigger tryin to adapt your infrastructure/app to Google App Engine compared to using a simple IaaS provider where you just have to spin up a virtual machine and use it like a normal VPS/physical server (if you dont want to integrate their API of course)