Let’s say you have a toy truck, and you angrily smash it into bits with a hammer (because you’re a troubled child). You would still have the same amount of truck that you started with, just in a different form than the original. But as you lift your hammer from the pile of toy parts following your last whack, you find that they’ve completely disappeared from existence.
You know this can’t happen because matter cannot be created or destroyed. So at the end of Terminator 2, when the T-1000 fell into the pit of molten ore, that glowing pool of lava had the T-1000’s mass and atomic components swimming around in there. The same thing happens when an asteroid gets sucked into a black hole. Even though we can’t see inside the black hole, we can tell matter isn’t being destroyed because the mass of the black hole increases by one asteroid. This is all making good sense, and then the universe makes like the Terminator timeline and just goes nuts. This happens when the black hole just up and evaporates, leaving scientists wondering where everything that got sucked in there went.
Black holes evaporate over time, and by “evaporate” we mean “disappear from existence,” along with everything it sucked in. The science behind this gets pretty complicated, but to put it as simply as possible, when black holes evaporate, they should emit a record of everything that was sucked in there written in the radiation. But according to Stephen Hawking, the black hole only gives off random heat energy.
In layman’s terms: If you find yourself in the unfortunate position of getting sucked into a black hole, you disappear completely from existence, as does any evidence that you ever existed.
“And this is possible thanks to a little principal called no goddamn clue. Sorry, guys.”
If they’d only thrown the T-1000 into a black hole, and the Terminators had to go back in time before that happened, he wouldn’t be in that timeline. Even with time travel, the universe wouldn’t know how to put him back together since the black hole that ate him disintegrated into nothing.