I think there is two reasons for this.
The first reason is because of the attack on Pearl Harbor. The Germans were obviously seen as a huge threat to the US, but they had not brought a fight over to US soil, while Japan did. Japan had attacked a few times (Pearl Harbor, Ellwood, Fort Stevens), and in turn, the US had developed a public paranoia against Japanese people.
The second is racial discrimination. Approximately 60% of those put into Japanese internment camps were US citizens. (This is the source I have for that, however I can't remember exactly what document it's from, I wrote a paper on this my freshman year of college 4 years ago.) Many people on the west coast had assumed that all Japanese (and Japanese-Americans) were spies to the Imperial Japan. However it is unlikely that any Japanese spy was Japanese. Japan did not necessarily trust their America counterparts, because they had left Japan. Japan would more likely recruit White or Black Americans.
Basically, more Japanese people were interred because of the attack that made the war feel much closer to home, and because they were Asian.
This is a great question, I would like to add a little to the previous comment.
As stated in a previous comment, 62% of Japanese internees were American citizens (30,000 were children). The presidential order was for all people with Japanese ancestry AND living on the Pacific Coast to be entered into internment camps. It might seem generous of the U.S. to only imprison Japanese living in the west coast, however, there were 127,000 Japanese U.S citizens at the time, and 112,000 lived on the west coast, so we interned a vast majority of ALL Japanese U.S. citizens. As a result of internment, jobs were lost, houses and land foreclosed or sold for a massive loss, and a rise in racism..The presidential order for these internment camps essentially “normalized” the idea that anyone who is Japanese is a spy. Even though 62% of them were U.S. citizens....
FUN FACT: Hawaii at the time had 150,000 Japanese Americans living there, but LESS THAN 2000 were actually placed in internment camps. This is because the Japanese Americans composed over 1/3 of Hawaii’s population, so total internment would result in destroying Hawaii’s economy.
Now let’s talk about the Germans soldiers, you know, the ACTUAL enemy of the war. All in all, 425,000 German POW were being held in 46 different states. They were allowed to work at farms or factories, AND the Geneva Convention required that they were paid American military wages. living conditions were good, and many German prisoners stated that the conditions were better than their civilian living arrangements back in Germany. Wine and beer were sometimes offered, and the prisoners were allowed to mail letters to their family back in Germany. the German POWs were even allowed to leave the camps without guards, “on the honor system” often visiting restaurants or enjoying the outdoors, and sometimes they would fraternize with American women, although that was not specifically allowed. These are hostile German soldiers that we are talking about, POWS captured during war...
To reiterate, U.S. CITIZENS, who happened to be Japanese, were treated FAR worse than actual Nazi’s that we captured during battle.
These are the facts, and based on the evidence, I would surmise the reason Japanese Americans were treated far worse than German prisoners is because:
Japan was the first hostile power in WW2 to actually attack the United States with massive casualties (2300 U.S. soldiers dead, massive loss of naval ships). Pearl Harbor was considered such a diabolical act, because it occurred without a declaration of war, and we had not even entered the war yet.
Pre-existing racism against foreign nations due to the rise of nationalism. Racism has always been around.
The Presidential order to intern all Japanese Americans solidified and almost encouraged the rise of Japanese bigotry. If the President is worried that Japanese Americans are spy’s, then the public will worry as well.