It's the same reason for not having an autopsy, even when apparent homicide. There are organizations the are always busy making sure that no autopsy is done on a Jewish Niftar.
@ChaimMoskowitz I understand why to a non Jew this would seem wrong, but you have to understand, it's not that we don't donate because we're afraid that a non Jew will get the organ (there are Jews donating kidneys altruistically to non Jews all of the time, I have intimate knowledge of that, I was very close to doing that myself), it's simply because we treat our deceased with utmost respect (we also insist on burying ASAP out of respect).
@Redbull3 if this makes you embarrassed to be Jewish, you've got many other Judaism issues...