I think it's because you can't chain BBtD to an effect that has yet to determine what player is gonna take the damage. It has to be 100% certain that you are the one who is going to receive it.
The effect of this card is tossing a coin. If you decided not to negate it, then it will resolve as tossing a coin.
You could, however use something like rainbow life to chain it.
Yes, the coin toss is the problem here. You always have to chain cards like BBtD to the activation but at this point, the coin hasn't been tossed yet and it's unsure who will take the damage.
True, but still so many of this hidden rules, beacuse how they write the text into the cards. In the end this is why we love and hate the game at the same time.
^x2
That's actually easy to answer. You're putting another monsters above them which means you can't see the material anymore. If you can't see it, it doesn't exist.
^In the anime the xyz material itself became some orb that surrounding the monster. It original form doesn't exist anymore, explaining it's monster effect doesn't valid anymore on the field, unless it has connection with the xyz monster itself