And how does the Thing survive long enough in an acid environment to take over? It's a built in defense, hell with a little exposition from a human character we might find out it's a natural defense that developed to survive the Things. They could be from the same planet.^ Remember what I just said. A Thing just has to get close enough to infect a Predator, and that only takes a few seconds.
Acid blood doesn't matter - even if the Predator manages to fight back and kill the Thing that infected it, it's still doomed, because Thing infection is irreversible.