[d1sxeyes]

It’s a good question. I’m not sure of exactly the answer, but I suspect the answer is similar to the answer “why do Americans speak German with an American accent?”

If you learn how to pronounce specific vowels, consonants, etc. in a particular way, it takes a LOT of effort to learn how to pronounce these in a different way. You can approximate to a good extent, but most researchers say that if you don’t develop this skill as a child, you won’t ever be able to pronounce things in a way that sounds like a native speaker.

Presumably, the models have been exposed to significantly more American accents than other accents, and learning how to pronounce phonemes with subtle differences without accepting a “close enough” approximation is a big challenge, especially given that there is already a threshold for acceptability at which level you can still sound like a native English speaker.