The author of Bowling Alone, Robert D. Putnam, is an American, and so it's neither unreasonable nor surprising that he should focus his attentions on the United States.
But more importantly than that, his America-centric focus serves the function of illustrating a phenomenon that, though far from exclusive to the United States, is nonetheless exemplified most strongly by recent trends in American society.
The decline of what Putnam calls social capital has been observed in other developed countries. But it has been much more acute in the United States. Here, as Putnam sets out to prove, trends of socialization have led to a noticeable diminution of the connections of trust that once existed between individuals in society. Where once people were brought together as part of a common endeavor, now they tend to stay in their lane, so to speak, separating themselves from others in an increasingly atomized society.
One could argue that Putnam has chosen well in deciding to focus his study on the United States. More than other developed countries, America has a strong tradition of individualism, which tends to militate against the development of collective experiences.
Although such experiences have, of course, existed throughout American history, they tend to be the product of crises such as wars and terrorist attacks such as those that took place on 9/11. At other times, however, American society remains largely atomized, with the tradition of individualism reasserting itself with renewed vigor each time a galvanizing national crisis subsides.