We've had to remove a number of posts with people sharing what they like on their pizza instead of pepperoni and/or challenging the OP's assumption. Please, if you want to discuss your favorite pizza, that is for another subreddit.
As for the fact that Pepperoni is the default topic, "default" is up for debate, I agree, but I think we can all agree that "most popular" is not a very contentious observation, and supported by polling on the matter. So I would ask that further discussion be focused on pepperoni's popularity in the American pizza business, and not on what you like on your pie. Thank you!
(Had to look at my physical books). According to The Oxford Companion to American Food and Drink and January 1985 issue of Gourmet Magazine, pepperoni is an Americanized version of Italian salami. In the early 1900's Italian immigrants brought salami to the US, but traditional salami is a seasonal product that required carefully controlled climate and other conditions and it was difficult to replicate the same process in an urban environment that the immigrants had settled in. Armour meat company developed the technology to control climate and automated the process so they could be produced year round. The first reference to pepperoni in print is from 1919. The American pepperoni uses lactic acid culture and greatly reduced curing time. As a result it became the most available Italian-style cured meat in the US (particularly when pizza became popular in the US in the 50's) before more authentic Italian-style cured meat appeared. In addition due to its shortened curing time the flavor can be improved by heating prior to consuming (usually salami is eaten cold).
Hopefully I do this correctly, I'm a long-time reader but don't often post. I wrote an essay about immigration and the influx of influx of food culture a few years ago in my M.A. program. I'll provide some excerpts and some commentary seeing that there are no useful threads posted yet:
Many will argue that interest in southern/eastern European food did not begin until after our soldiers returned from WWII. There are some arguments, however, that the assimilation of ethnic foods began much earlier. A book entitled, The Grocer's Encyclopedia, published in New York in 1911 provides, a detailed look at the demand for products during the time period. It was essentially publish as a guide for large, commercial markets to understand the differences between different ethnicities and their food. The volume, in addition to containing Jewish Dietary Laws contains list of more than 500 of the most popular food words and concepts translated into German, French, Swedish, and Italian.
Around this time there are also a few other cookbooks that begin to get translated into English for the first time. One volume of particular interest was a compilation entitled With A Saucepan Over The Sea; Quaint And Delicious Recipes From The Kitchens Of Foreign Countries. This volume provided signature recipes from almost every country in Europe and even singled out different ethnic groups by providing Jewish recipes from both Poland and Germany. This demonstrates both the diversity of ethnic groups and also the interest in redefining American food culture with an international palette.
The increase in the availability of ethnic street food provides some additional evidence that eastern/southern European food was becoming more popular. During the "gilded" and "progressive" era a large percentage of immigrants were engaged in street vending. New York City during the time period actually made special laws to allow street vending to occur. With the large amount of eastern/southern Europeans engaged in the food market, they had a large impact in the types of food available on the street for regular consumption. (Bhat, R.V. (Editor). Street Foods (Vol. 86, Wrund). Basel, Switzerland: Karger Publishers, 2000. p 27.)
That being said, pepperoni pizza is an excellent example of a sociological theory referred to as (I believe) the diffusion and reformulation of culture. Just like McDonalds in India has more vegetarian options as they have reformulated the diffusion of fast food culture to meld with their own. Pizza was reformulated to meld with existing American culture. "Americanization" was already taking place during this time for immigrants. This americanization movement also extended to food and diet. "As the waves of immigrants from Europe and elsewhere poured into the great cities of America at the end of the 19th century, earnest American social workers, medical personal, home economists, educators and others became increasingly concerned with the newcomers' diets and health. Numerous studies were undertaken to learn about the immigrant's foodways, mostly in an attempt to change them." (Feeding America Project,Michigan State University, Anne-Marie Rachman, Digital & Multimedia Center, Michigan State University Libraries). Immigrants were actively encourage to change their dietary habits to "healthier," American options.
I would assume that pepperoni was one viable option for the reformulation of pizza culture to the U.S. Dried meats were utilized in both the U.S. and Europe. Pepperoni has a relatively long shelf-life and can be used on pizza in the winter when other toppings may not have been available.
I went off subject for a little bit but that's the best answer that I can come up with. In simple terms, pepperoni was a culturally acceptable topping where American's could eat it and feel like they're exploring "foreign" food without getting too far off the beaten path.