Sadly, I would bet that it's the jewish lobby that pushed a lot of countries to oppose this. They have this need to make the holocaust be the worst thing that has ever happened to any people in the history of time.

The holocaust certainly bad, it's among the worst mass killings of all time, and the fact that it happened in relatively modern times makes it worse because the world generally isn't as brutal as it once was. Is it worse than the Mongol invasions, which may have killed more than 10% of the entire world's population at the time? Worse than historical wars in China which killed tens of millions at a time when the entire world's population was under 200 million? Where would you rank African slavery in that? Is it less bad because fewer people died, or worse because there are things worth than death? I don't really think it should be something you rank at all. And, I'd also oppose any attempt to rank any of them as "the gravest crime against humanity", because what's the point of that?