* In-house development teams have more agility in standing up new services, since they can just use EC2/ECS/one of the other 1500 container runtimes AWS has, rather than having to wait for someone to provision new physical servers for them.
* An extensive suite of complementary products are available from cloud providers, it's not just about being able to start a VM, in many cases you don't even need to because you've got Lambda, and the various managed services.
* Less training/hiring overhead, since people who can use AWS are pretty much a commodity at this point, whereas people who can manage a mainframe are an increasingly rare breed.
* Redundancy becomes easier. As does data locality, let's say Singapore declare all services delivered in Singapore must be hosted there. That's a lot easier to handle if you don't have to go and build/acquire a data centre first.
* The aforementioned 400 million dollars per year.