It's presumably American Gods because Neil felt that was the best setting for the story he wanted to tell. Neil being British is irrelevant. He has described it as being a book about immigrants, so it's entirely appropriate to have multiple nationalities reflected in the cast.
And yet your 15% statistic is more than a tad inaccurate:
If you want to be racist you can go ahead and exclude people that moved to American as children - just because Pablo Schreiber and Yetide_Badaki have both lived, studied and worked in the country for over 25 years, they're clearly not proper USians. Here's the pie chart that puts them entirely where they belong:
That puts it at 48% of proper Americans playing the significant characters. In a series based on a book about immigrants. Very curious.