Because table lookups don't vectorize. You could try to use vectorized gather (VPGATHERDD [0]), but so far in my experience it's been just as slow as using scalar code. (Then again, even gather instructions operate just on 32-bit data, so it wouldn't help anyways.)
So to perform multiple operations in parallel per core (SIMD), you'll have to write some code to perform the "lookup" transformation instead.
