How did tofu become popular in the US and Westernized countries? The history of tofu is about as interesting as the food itself so finding articles on the subject is extremely difficult.
I wouldn't call it "popular"; it has an established presence, but it's a niche food rather than a popular food. The popularity that it does have is from two different paths:
Its use in Asian cuisine and the rise in popularity of Asian cuisines in the West. There have been a number of studies and books on the growth in popularity of Chinese cuisine in the USA (and elsewhere in the West), e.g., J.A.G. Roberts, China to Chinatown: Chinese Food in the West, Reaktion (2002).
Its role as a health food and meat substitute. In the USA, this was pioneered by Seventh-Day Adventists in the 1920s and 1930s.
The soy-related question of soy sauce in the West was asked recently, and I pointed the OP to a nice soy history resource:
http://www.soyinfocenter.com/index.php
Relevant to your question, they have a set of history of tofu pages; US history is covered in pages 4-6. For more detail, they also have a 4016 page pdf document History of Tofu and Tofu Products (965 CE to 2013) which you can download from http://www.soyinfocenter.com/books/163 which might satisfy your appetite for tofu history for the time being.
It doesn't answer your question, and I hope it's permissible, but before you dismiss tofu as uninteresting may I submit this article from Bon Appetit on artisanal tofu making in Kyoto.