In the case of DNA, because it's shared with relatives and descendants who might be still alive. In Hitler's case, that might not be that much of an issue, but you were talking about dead people in general.

If your parents are dead, and thus they get DNA sampled, that information gained is good enough to positively identify DNA traces of all their children.

Remember how they caught the Golden State Killer? They put a DNA sample into the genetics website GEDmatch and found a few of his distant relatives. They then used publicly available family history records to construct a family tree that included all of these matches. That allowed them to narrow down the suspects to two people. One of them could be ruled out by DNA testing a close relative, which left the last one. They then took a DNA sample from his car, which was a match and that's how they got him.

Using that kind of stuff to catch killers is likely a good use of the technology, but there's quite a few nefarious things a state could do with a DNA database of all dead people.