The failure to mention Jews, or any other group targeted by the Nazis, was not accidental according to the Administration.
It is true that millions died under the Nazis who were not Jews. They included targeted ethnic groups like Roma (Gypsies), homosexuals, left-wing dissidents, people with disabilities, etc. But it has long been a trope of Holocaust deniers and white nationalists to insist that Jews were only incidentally targeted.
In any case, there are no word limits in presidential statements. A more logical and worthwhile approach would be to note the various groups who were victimized. This isn't accidental. The new administration is riddled with anti-Semites and those who want to cater to anti-Semites.
Marshall blames alt-right godfather Steve Bannon.