Did racism play a part
I'm prefacing this answers with three points. 1: I'm much more familiar with the internment of the Japanese-Americans than of the German and Italian-Americans, so this answer is more from the perspective of why Japanese-Americans were detained in such large numbers than why German and Italian-Americans were not detained. 2: Most of my information comes from Roger Daniels' excellent book Prisoners Without Trial. 3: There was detainment of German and Italian-Americans during World War II.
Before the war, there had been ongoing tensions between the Asian immigrant population and the native White population. Asian-Americans had served both as cheap labor and as convenient political scapegoats in the form of the Yellow Peril. In fact, fears about Chinese immigration would result in the passing of the 1882 Chinese Exclusion Act which barred the immigration of any Chinese Immigrants for 10 years. This act would be renewed multiple times until its final repeal in 1943. When it was halted, the Chinese-American population was about 104,000. Coincidentally, it is around the time of the Chinese Exclusion Act's introduction that significant Japanese immigration began, with the Japanese-American population rising from 3,000 in 1890 to about 127,000 in 1940, about 94,000 of which lived in California. At this point, Japanese-Americans made up less than a tenth of a percent of America's population and about 2% of California's population. This is not a large amount by any measure. Furthermore, the Naturalization Act of 1790 and its 1870 Amendment restricted naturalized American citizenship to only White and Black people, so only the descendants of Japanese immigrants even had the opportunity to be citizens and participate in American politics. The Immigration Act of 1924 would ban all immigration from Japan, further limiting the Japanese-American population.
Despite this, the Asian population was seen as a threat, and the rapid rise of Japan further intensified American's fear of this population. Amplifying American anxieties was Japan's victory over Russia in the 1905 Russo-Japanese War. Prisoners Without Trial includes some telling headlines such as:
It's in this background that in 1941 the Japanese Empire would stage a surprise attack on the American naval base in Pearl Harbor. Germany had succeeded in a lightning assault a quick conquest of much of Europe, and fears of Japanese collaborators, the so called "fifth column" grew. However, it should be noted that there is no evidence of Japanese-American collaboration with the Japanese Empire. In fact, a contemporary report to the president stated that "widespread sabotage by Japanese is not expected," and a naval intelligence report stated "that better than 90% of [second generation Japanese-Americans] and 75% of original immigrants were completely loyal to the United States." In spite of this, there was widespread fear of the fifth column, bolstered by Japan's rapid conquest of the Philippines, which was at the time an American possession. The Secretary of the Navy blamed the damage to Pearl harbor on "treachery" and that it was "the most effective fifth column work that's come out of the war, except in Norway." Similarly, the day after the Pearl Harbor attack the Los Angeles Times published that "We have thousands of Japanese here... Some, perhaps many ... are good Americans. What the rest may be we do not know, nor can we take a chance in the light of yesterday's demonstration that treachery and double-dealing are major Japanese weapons."
To be clear, the Japanese Empire wasn't a non-threat. After the attack on Pearl Harbor, Japan staged a quick campaign, overrunning America and her allies' possessions in the western Pacific. On the same day as the Pearl Harbor attack the Battle of Wake Island also began, and it would be taken on December 23rd. Guam fell earlier on December 10th, and Hong Kong on Christmas Day. Similarly Malaya, Singapore, and the aforementioned Philippines would be taken in the first half of 1942. At the time, there was a widespread belief that Japan could stage an attack on the American West Coast, and that Japanese-Americans could assist an invaded Japanese Army. This belief was backed up with the Ni'ihau Incident, where a Japanese pilot from the Pearl Harbor attack crashed onto the Hawai'ian island of Ni'ihau and took several hostages with the assistance of a Japanese immigrant and was assisted by two other Japanese-Americans. While this incident wasn't reported on a wide scale, it was noted by a naval report from January 1942, citing the incident as showing that Japanese-Americans thought to be loyal could collaborate with a Japanese invading force. What role this incident would have on the passing of Executive Order 9066 and the subsequent internment of Japanese-Americans is uncertain. Greg Robinson's A Tragedy of Democracy states that "there is no evidence that the Niihau Incident influenced later policy." William Hallstead's article on the incident states "there can be no doubt that the Niihau event influenced [the internment.]" From the newspapers of the time, it appears it was overshadowed by the events in the west Pacific, with some papers reporting it and others not mentioning it at all. Regardless, Executive Order 9066 would be issued on February 19, 1942.
Executive Order 9066 did not target any ethnicity specifically, but it would be used to "relocate" nearly the entire Japanese-American population to internment camps in the interior of the United States, and the rapid and indefinite evacuation of Japanese-Americans deprived many of their property. The effort and resources to move and accommodate well over 100,000 people was enormous, and the last detainee would be evicted in 1946.
That's an abridged overview of Japanese-American internment, but what about German and Italian-American internment? Sadly, I'm not familiar enough with those subjects to give a full explanation of them. During World War II about 11,000 German-Americans and 2,000 Italian-Americans were incarcerated, figures particularly astonishing when one considers those populations were many times greater than the Japanese-American population. The primary factor distinguishing these cases was race, as previously mentioned there was no evidence of a Japanese "fifth column," so Japanese-Americans weren't any more dangerous than their European counterparts.
In summary: The scale of the incarceration of Japanese-Americans into internment camps was primarily motivated by anti-Japanese racism.
Edit: Modified conclusion to be more clear.
Edit 2: Added a paragraph on the progress of the early stages of the Pacific War and the Ni'ihau Incident.