They tend to cluster in certain metropolitan areas in the Rust Belt (St. Louis, Pittsburgh, Chicago, Cleveland, the latter two of which used to have regular JAT flights in 1980s) that have been hit hard by deindustrialization.
Source: I'm from Cleveland, aunt married one of the (many) Slovenian-Americans there
Those cities sound familiar from high school geography. We were told that Pittsburgh was the city with the world second largest Croatian population. Not sure when that would have been true but cool nonetheless.
Source: I'm from Cleveland, aunt married one of the (many) Slovenian-Americans there