Considering approximately only 10k German Americans and under 2k Italian Americans out of the possibly millions were interned. While over 110k of the 130k Japanese Americans living in mainland US ended up in the camps.
Is this an indirect result from the Pearl Harbor attacks since German and Italian forces primarily focused attacks in the European and African continents? Was it complications with establishing German or Italian ethnicity amongst Caucasian Americans?
You actually sort of have it already!
German Americans both made up a prohibitively large portion of the population to intern in 1941, and were spread out over the vast majority of the nation. Simply put it wasn't worth the effort, and if they really wanted to the population could destroy the economy. But also many signs of outright German cultural identity had been toned down during WW1. Laws had been passed doing everything from banning German language newspapers, to the teaching of the language in school, to renaming towns after 1917. By 1941 there was simply a much recued German "identity" than in 1917 or compared to Japanese Americans.
The Japanese American population then had several things going against it. 1 is that it was relatively concentrated in California and the rest of the West Coast. 2 is that while mostly 2nd and 3rd generation Americans, so the children and grandchildren of the original immigrants) they still retained a strong community and cultural identity. 3 is that this was helped along in no small part by both fear and anger on the part of local group of the "yellow peril". While the strength of the Japanese Empire meant that their immigrants were handled more delicately compared to China, and limits on immigration ere only by mutual agreement, it also sharpened fears. Simply put the white population kept Asians separate, and then that made it easier to identify them as being separate.
The Nihau Incident, in which a Japanese flyer crash-landed on a small Hawaiian island after the Pearl Harbor attack and was aided by two Americans of Japanese descent, also helped persuade Yanks that even U.S. citizens of Japanese descent couldn't be trusted. There's a good book by Allan Beekman on the topic.