They are probably starting with the Holocaust victims because there are likely better records that can be cross-referenced. The Nazis were meticulous record-keepers. The recorded evidence they failed to destroy proved pivotal in several war crimes convictions.

I don't think anyone is gatekeeping the technology or saying other genocides and massacres aren't important. I think they're probably starting with something they think has a higher probability of making accurate matches, therefore improving subsequent use of the technology.