Your link says best estimates are that several hundred thousand Romani were killed by Nazis, which is terrible and should be recognized and memorialized, as it is indeed at the U.S. National Holocaust Museum.
When you get right down to it, the Nazis were probably ultimatelyresponsible for 20 to 40 million deaths, most of whom were Slavic peoples of the Soviet Union.
The reason to hold the holocaust out as a poster child of a crime of humanity is because of the scale, nature, and single-mindedness with which a national policy of mass murder was pursued. I mean, even as the German armies were collapsing in the face of the relentless Red Army and western allies, the Nazis still continued to devote valuable military, strategic, and transportation resouces into hunting down Jews and other "undesirables", transporting them to death camps, and murdering them.
Those were resources a rational military commander would have devoted to resisting the onslaught of the allied armies and air forces.