I actually didn't think of the answer to this question for months, which is enough to disgrace my biochemistry professor, really. The reason is so simple, so beautiful, that it really should have popped into my head long ago. Alas, it did not. But yesterday it finally did, so on the off chance you want to know why you can't block synthetics, keep reading. I don't get really technical.
First, a bare-bones lesson on proteins. Proteins are long chains of amino acids curled up on themselves. The the twenty-plus amino acids all have different things attatched to them. Some of those things can be very attracted to each other, which makes them stick together and affect the shape of the protein depending on where amino acids are on the chain. That's important because, from human hair to merino wool to fuzzy angora, fur is just a bunch of proteins strung together.
Now, a little about synthetic fibers. Synthetic fibers are polymers, or long chains of molecules. Like proteins, actually, and many other things both natural and manmade. Chemically, they are both organic, or carbon-based. To look at the molecular structures or even just their molecular formula, there isn't much difference between some proteins and certain ploymers. That's why acrylic is so widespread in garment making: it resembles wool on a very tiny level.
The differences that are there, though, have a large impact on blocking. The molecules hanging on a protein can bond to each other in a couple different ways. These ways can be affected easily by heat and water. Ever curled your hair? If you've done it without resorting to getting it wrapped painfully on uncomfortable plastic rollers and doused in smelly chemicals, you probably used a curling iron or rolled it when it was wet and let it dry like that. Both methods temporarily alter the chemical bonds along the protein strings of your hair, which then alters their shape.
The same happens when you block natural fibers. Wet blocking and steam blocking do the exact same thing to the fur your item is made of that it would do to your hair: it forces the temporary rearrngement of bonds. Acrylic, on the other hand, no matter how much it feels like wool, does not have the specific chemical structure to form bonds with itself along the polymer string. It just slides.
So that's why you can't block acrylic. And why you have to reblock things after they get wet. Now here's the question I have: has anyone ever tried to give a wool garment a perm to set the blocking for all time? If they did, I bet it was really smelly. Wet wool + perming chemicals = YUCK!