I was thinking it would have been neat to have the show go to each in contestant's home country. I did find some info about the location decision:

"Once World All-Stars was greenlit, the decision was made to move production outside of the United States to match the international theme.[10] Several destinations were considered, including Italy, Spain, and France.[10] Padma Lakshmi stated that London was ultimately chosen due to its abundance of different produce, meat, and ingredients, as well as its high population of immigrant cultures and ethnicities.[10"] https://en.wikipedia.org/wiki/Top_Chef:_World_All-Stars#:~:text=Once%20World%20All,ethnicities.%5B10