unfortunately, there is no public roadmap related to a rollout Stadia to new countries and regions. You might want to follow the Stadia team on Twitter to stay up to date. (https://twitter.com/googlestadia)
Why doesn't Google have any plan on expanding Stadia to other regions such as South America?
Honestly, it seems not to be a smart decision since there will be WAY more users there than on the developed countries the service is already available in (where everyone can afford any gaming PC or videogame).
He didn't say there wasn't a plan. He said there's no public roadmap. Google has detailed plans for lots of things. But, they tend to keep most of their plans secret until they're closer to releasing a feature. That said, Stadia requires a lot of physical infrastructure to be built in each region, and they'll have to do the math to be confident that investment will pay off. They can't do that everywhere all at once.