There were three (!) times more German immigrants than Italian immigrants. Polish and French immigrants were next in number to the Italians, but even in Chicago, where the Poles are omnipresent, Polish restaurants can be found only in the ethnic enclaves, while Italian restaurants are everywhere. I like Italian cooking, but this popularity has little to do with the quality of food per se (e.g., I like Spanish and Greek cooking even more). There is something about Italian restaurants that clearly does not apply to other restaurants. The reason is an intersting bit of history.
Italian food suddenly became popular during the early years of Prohibition. The law had an important omission: while wine could not be sold publicly, it could be made for "domestic consumption." Italian-Americans were much better than the others at such medium-scale urban home wine-making. Wine is not whiskey: you need to drink a lot to become drunk, so you want to have a meal to wash it down. Guiseppe Prezzolini put it this way: "In more or less dimly lit basement speakeasies, Americans could fine wine, more or less palatable, made by an Italian, more or less legally. And what would the Italian serve to go with his wine? Thus proper spaghetti was infromally introduced to millions of Americans." (Spaghetti Dinner, 1954) Because so many spaghetti houses became semi-legal speakeasies, the frequenters of such places (the majority of urban Americans) rapidly acquired taste for spaghetti.
...When Prohibition went into effect on January 16, 1920, it put hundreds of restaurants and hotels out of business and destroyed the last vestiges of fine dining in the United States... The American wine industry, unable to sell its wines legally, quickly turned its vinyards over to juice grapes. But only a small portion of the juice from the grapes was marketed as juice. Most of it was sold for home-brewed wine. Needless to say, this home brew was not usually a sophisticated viniferous product, but sales of the juice kept many of the vineyards in profits throughout Prohibition. The bad alcohol, the closing of fine restaurants, the sweet foods and drinks that took alcohol's place, the artificial flavors that were used to simulated alcohol, all these things could not help but have a deletrious effect on the American palate. Prohibition, with its tremendous impact on the eating habits of the country, also had a great deal to do with the introduction of Italian food to the masses. Mary Grosvenor Ellsworth, in Much Depends upon Dinner, (1939), said this about Prohibition and pasta: "We cooked them [pastas] too much, we desecrated them with further additions of flour, we smothered them in baking dishes and store cheese. Prohibition changed all that. The Italians who opened up speakeasies by the thousand were our main recourse in time of trial. Whole hoards of Americans thus got exposed regularly and often to Italian food and got a taste for it. Now we know from experience that properly treated, the past is no insipid potato substitute. The food served in the speakeasies--with Mama doing the cooking and Papa making the wine in the basement--was not quite the same as the food the Italians had eaten in the Old Country. Sicilian cooking was based on austerity...But America was rich, and protein rich country, and the immigrants were happy to add these symbols of wealth to their cooking--and happy that their new American customers liked the result. Meatballs, rich meat sauces, veal cutlets cooked with Parmesean or with lemon, clams ctuffed with buttered herbed crumbs, shrimp with wine and garlic, and mozzarella in huge chunks to be eaten as appetizer were all foods of abundance, developed by Italian-Americans." http://www.foodtimeline.org/fooddecades.html
Of course, German-Americans also knew how to brew beer for "domestic production." Why did not they join the Italians? This is another peculiarity of Prohibition:
...Even before the 18th Amendment was ratified, about 65% of the country had already banned alcohol. America's entry into WWI made Prohibition seem patriotic since many breweries were owned by German Americans. Wayne Wheeler, lobbyist for the Anti-Saloon League, urged the federal government to investigate "a number of breweries around the country which are owned in part by alien enemies." In December 1917, Congress passed the 18th Amendment. A month later, President Woodrow Wilson instituted partial prohibition to conserve grain for the war effort. Beer was limited to 2.75% alcohol content and production was held to 70% of the previous year's production. In September, the president issued a ban on the wartime production of beer. National Prohibition was defended as a war measure. The amendment's proponents argued that grain should be made into bread for fighting men and not for making liquor. Anti-German sentiment aided Prohibition's approval. The Anti-Saloon League called Milwaukee's brewers "the worst of all our German enemies," and dubbed their beer "Kaiser brew." http://www.digitalhistory.uh.edu/database/article_display.cfm?HHID=441
German-Americans did not dare to become involved, because they were at the center of Germanophobia campaign fanned by the Progressive/temperance movement. It went much further than renaming sauerkraut and hamburgers into "liberty cabbage" and "liberty sandwiches" (yes, freedom fries have a precedent); it was unrestrained jingoistic paranoia. Wheeler (the chief architect of Prohibition) himself pressed the Senate into dissolution of the German-American Alliance that was poisoning Americans with their demonic Bier. In these German immigrants, the homegrown American proto-Nazi found the socially-acceptable outlet of their fury, and it was the last element they needed to institute their version of hope'n'change thereafter:
...German-American schools and newspapers by the thousands were forced to permanently close. In cities and towns across the nation, libraries burned their German-language books in public burnings. The officials of German-named towns that had been founded by German-Americans were intimidated by county, state, and federal government officials into anglicizing their names, and into destroying all traces of their German heritage. In cities across the United States, German-sounding street names were banned. Many families with a German-sounding last name changed their surname. Newspapers in New York and other places published lists of inhabitants names and addresses, labeled as Enemy Aliens, thereby inviting neighbors to hostile actions. As the public atmosphere became increasingly hysterical, vigilantes burned "pro-German" books, spied on neighbors, and attacked and murdered immigrants. Anti-German tension culminated on April 4, 1918, in the brutal lynching of German immigrant Robert Prager, a coal miner living in Collinsville, Illinois, who was accused of making "disloyal remarks". In June 1918 a bill was introduced in the House of Representatives by Representative John M. C. Smith with the aim to wipe out German names from the map of the United States. (Wiki)
That's why we have many Italian and so few German restaurants. The food preferences of modern Americans were set during the bottleneck during the Prohibition Era, when the Italians possessed the crucial skills while the Germans were noncontenders.
So it became spaghetti.