Hi @metzli,
That's a good question. The authors used a collection of strains, collected at various locations. The paper talks about this and even shows a figure (see below). As you can see, many isolates are from the US, but there are also a significant number from other countries in Europe (Belgium and Denmark). I think this is more a historical thing though. Brettanomyces species are found nearly everywhere. It is just a matter of where you wish to look and isolate them from.
Thank you for your comment and vote!
Best wishes,
Edgar.
Figure 1 from Colomer et al. (2020)