Simba I think rather than choosing only to do the US due to the US having a large population (although not the largest by a long way, India and China have many times higher population and many times more people who are blind), it was instead more than the USA had to be addressed before other countries, as there was a regulatory deadline involved for USA and not for anywhere else.
I can't speak for PlayStation obviously, but I'd personally be surprised if TTS isn't expanded further in the future. There's precedent, that's what happened with Microsoft.. Not just Xbox, but that's the pattern seen with other tech too, such as Windows Phone - address the country with the regulations as first priority, then look at expanding out to other countries.
Which of course is a great thing, proves that even if regulations are a bit of extra motivation to get things moving they aren't the sole reason why companies do this kind of stuff.
And also, very true about why they started with the US, or at least North America, first. Here's hoping for many great things from Sony... as cynical as I've sounded, when Sony actually have updated it, it's been pretty big every time... so here's hoping they don't just do enough to satisfy the new addition to the ADA. Though I think it would've been more productive if that new regulation stated how much of any given hardware should be accessible. 