Primarily, as /u/thelazerbeast points out, because there were simply too many German-Americans (and German citizens living in America), too well-assimilated into the fabric of American society, for mass internment to be feasible or without serious consequences.

From Personal Justice Denied, the report of the Commission on Wartime Relocation and Internment of Civilians (p289):

The American population of German descent in 1940 was so large that any major program of exclusion or detention would have been very difficult to execute, with enormous economic and political repercussions. In 1940, 1,237,000 people of German birth lived in the United States... Further, if one considered the children of families in which both parents were German-born, the number of Germans reached 5 million and, counting families with one German-born parent, the number rose of 6 million. A population of that size had political muscle; the industrial northeast, the midwest and the northern plains states all had substantial German American voting blocs. Radical measures such as exclusion or detention would have carried a very heavy political cost.

There's a whole chapter on Germans and German-Americans in the report which analyses your question in great detail — there's also a good argument to be made about systemic racial prejudice against the Japanese (which simply did not exist in the same way directed towards Europeans.)

Important to acknowledge, however, that there was some limited internment of Germans in America — about 11,000 in total during the war, or 36% of the total interned under the Justice Department's Enemy Alien Control Program. From the report (emphasis mine):

Immediately after the Pearl Harbor attack, the FBI picked up Axis nationals whom they suspected, frequently on the basis of membership in suspect organisations. By February 16, 1942, the Justice Department had interned 2,192 Japanese, 1,393 Germans and 264 Italians. (p284)

There were also forced deportations/pseudo-extraditions of around 4,500 Germans from Latin America to the United States, where they were interned on the orders of the State Department's Special War Problems Division.

Actually, some German citizens and some German-Americans were interned, but nowhere near the same scale as Japanese Americans and Italian Americans were, and this was due in large part to the sheer number of Germans and Americans of German descent (a plurality is many states, especially during this period in history), which would have made complete internment quite difficult.

Instead, the government arrested and interned select individuals considered to be national security threats from the coastal areas, and moved them to camps inland. That most ethnic Germans, citizens or otherwise, already lived far into the US mainland (Pennsylvania, Minnesota, Wisconsin) thus affected the number of those interned.

A good book on this is Stephen Fox's book Fear Itself.