The Jews outnumbered any other group in the actual extermination camps, though about a million Gypsies/Romany were also exterminated in them. Very likely far more Slavic speakers than Jews were killed (Not in death camps as such, though the German PW compounds in the East might as well have been death camps, because the Nazis weren't too concerned with actually feeding prisoners; at the same time, the ruthless exploitation of the population and purposeful destruction of the intelligentsia in occupied Slav lands like Poland, Belarus, and Ukraine very likely killed at least as many Slavs as the Holocaust did Jews).
But really it comes down to European Jews having a voice to tell their story, and a receptive audience to tell it to, in the West, while for various reasons none of the others did.