Probably because many treatment options can cause harm. So no researcher wants to take the very real risk of harming another human. If you harm an animal unintentionally, and in the name of trying to do something good, not many people will question it. But if you kill a paralyzed patient, or make him suffer excruciating pain, you're likely going to feel remorse and his family is going to demand an answer.
I think the social taboo is self-reinforcing argument. Animals have the same rights as humans. Indeed, we are animals. Those who draw categorical lines to justify their morose experimentation ought to be judged in the same light as someone who would cage a human boy and deliberately sever his spinal cord in the name of a higher good. It's the same. I don't condemn the science, but I do condemn the hypocrisy.
>Animals have the same rights as humans. Indeed, we are animals
By that reasoning, oughtn't we grant the same rights to a sea sponge?
It's not clear to me that animals, as a general class, deserve any particular care or rights beyond what we might extend to living creatures in general. So far as we grants rights to animals, the clear criteria seems to be complexity (especially of the intellectual sort) and closeness to humanity.
