Because Google has the money to build 10 different versions/iterations of Gemini and can essentially force one to work. They have most people's data and most people use them for mail/search/browser/maps as well.
In my opinion though this is a race to the bottom rather than a winner takes all situation so I don't think anyone is coming out ahead once the dust settles.
Does it matter? Microsoft won by default with Teams because it actually turns out no one cares about chat or even has a choice in it: employees use whatever the company picks.
If you're going to say "other than the US" then you've got to say at a minimum "other than the US and China", but really "other than the US and China and Japan and Korea and Taiwan and Thailand and Russia and most of Central Asia".
Only mentioning the US is wildly americentric even by HN standards.
The tech behind wave eventually made its way into Google docs though and pioneered collaborative document editing, so wasn't a complete failure even though the product itself was killed.
No comment on Google+, Google has a storied history of failure on any kind of social media/chat type products.
Where Google wins is just simply having enough money to outlive anyone else. As the saying goes "the market can remain irrational longer than you can remain solvent" In this case, Google is the market and they can just keep throwing money at the wall until OpenAI, Anthropic, etc. go under.
In my opinion though this is a race to the bottom rather than a winner takes all situation so I don't think anyone is coming out ahead once the dust settles.