Because Hitler and his party found the Jewish people inferior to their "german pure blood race". The Jewish were successful and rich, well the majority and had a lot of property they owned, and Hitler blamed them for the Great Depression coming to Germany and wanted them dead along with other inferiorities he considered. The Aryans (indo-europeans) were blondes and blue eyed and not most germans fit this description, and even though, they weren't excuted only because they were german citizens. However the Jewish and non-jewish poles, homosexuals, disabled, mentally ill and others were prosecuted to concentration camps and death camps. Basically because Hitler thought they were inferior.

Hitler was a fanatical antisemit and there were more of them in his party. Propaganda and a broadly spread antisemitismus made the rest. Most of the german did not want a genocide and did not know (or: did not want to know) that it was taking place, at the time. But however, they made it possible. Source for further information: http://en.wikipedia.org/wiki/Holocaust

Genocide was "good business". Firstly, it unified Germany against an internal foe... "us" against "them". The rampant hatred and distrust of the Jews made it oh-so-easy to unify Germany against them. Secondly, the Nazi war machine soaked every penny out of the Jews that they could get... this done primarily in the Ghettos and completed in the Camps. I firmly believe that part of the reason for the Jewish genocide was financial gain. The Gypsies? We had no money. The Final Solution for the Gypsies was instituted in 1941. I guess Hitler saw that the Jewish "Final Solution" had so many "benefits" so he needed another lot. Gypsies were handy and hated. Easy targets. I also believe that he DID believe that his version of "Aryan" was superior. I do not know if he always believed so but, at the end, I firmly believe that he accepted his own dogma wholecloth. I believe that had he been successful in eliminating Jews, Gypsies, homosexuals, etc et al, he would have found another group to target. It just plain suited his means so well. Oh, guess what... Rromani ARE technically Aryan. How is THAT for a historical "kick in the pants".