I may be completely off here, but isn't this due to their underlying architecture decisions? That is, AWS from the start has kept all regions completely separate, so that problems in one region do not influence another. But GCP has has issues with failure across regions IIRC.
Having a software defined networking spanning across regions and failure cascades across regions are two different things. There's nothing preventing a vendor from presenting to you a single network, while they are actually distinct networks.
It is also potentially due to Google owing their private fiber backbone that connects all regions and as well as their software Defined Network that allow high bandwidth and low latency routing of packets across regions.
