The Google Cloud team had more incentive than other teams would to be an early adopter in dogfooding their own products, in this case App Engine specifically.

Ending up on Angular was likely a consequence of using app engine: all of the standard google frameworks for writing UIs have a server-side component to help deliver data and scripts to the client, and most of those frameworks would have been difficult or impossible to port to app engine (either because of the chunked transfer encoding limitation, or just the general awkwardness and annoyance of porting a framework maintained by a completely different team from borg to app engine).

The codebase probably started with something that looked like an app engine example app, transferring data and code to the client in the simplest way possible, but not the most performant or scalable way.